diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/GraphNodes/Actions/SearchGraphActionHandler.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/GraphNodes/Actions/SearchGraphActionHandler.cs
index 40f297bfa3b..8c0cc4ea1c4 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/GraphNodes/Actions/SearchGraphActionHandler.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed.VS/ProjectSystem/VS/Tree/Dependencies/GraphNodes/Actions/SearchGraphActionHandler.cs
@@ -108,10 +108,10 @@ private void Search(IGraphContext graphContext)
                                 cachedDependencyToMatchingResultsMap);
                         }
 
-                        cachedDependencyToMatchingResultsMap[topLevelDependency.Id] = topLevelDependencyMatches;
+                        cachedDependencyToMatchingResultsMap[topLevelDependency.Id] = topLevelDependencyMatches!;
                     }
 
-                    if (topLevelDependencyMatches.Count == 0)
+                    if (topLevelDependencyMatches!.Count == 0)
                     {
                         continue;
                     }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildExtensions.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildExtensions.cs
index 04177bfd830..401316d019a 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildExtensions.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildExtensions.cs
@@ -5,8 +5,6 @@
 using Microsoft.Build.Construction;
 using Microsoft.Build.Evaluation;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Build
 {
     internal static class BuildExtensions
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildUtilities.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildUtilities.cs
index 39072cfd818..772bbe37bb4 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildUtilities.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Build/BuildUtilities.cs
@@ -10,8 +10,6 @@
 using Microsoft.VisualStudio.Buffers.PooledObjects;
 using Microsoft.VisualStudio.Text;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Build
 {
     /// <summary>
@@ -54,7 +52,7 @@ public static bool HasWellKnownConditionsThatAlwaysEvaluateToTrue(ProjectPropert
         /// <param name="project">Xml representation of the MsBuild project.</param>
         /// <param name="propertyName">Name of the property.</param>
         /// <returns>Requested project property. Null if the property is not present.</returns>
-        public static ProjectPropertyElement GetProperty(ProjectRootElement project, string propertyName)
+        public static ProjectPropertyElement? GetProperty(ProjectRootElement project, string propertyName)
         {
             Requires.NotNull(project, "project");
 
@@ -70,7 +68,7 @@ public static ProjectPropertyElement GetProperty(ProjectRootElement project, str
         /// <returns>Collection of individual values in the property.</returns>
         public static IEnumerable<string> GetPropertyValues(string propertyValue, char delimiter = ';')
         {
-            HashSet<string> seen = null;
+            HashSet<string>? seen = null;
 
             // We need to ensure that we return values in the specified order.
             foreach (string value in new LazyStringSplit(propertyValue, delimiter))
@@ -223,7 +221,7 @@ public static ProjectPropertyElement GetProperty(ProjectRootElement project, str
         public static ProjectPropertyElement GetOrAddProperty(ProjectRootElement project, string propertyName)
         {
             Requires.NotNull(project, "project");
-            ProjectPropertyElement property = GetProperty(project, propertyName);
+            ProjectPropertyElement? property = GetProperty(project, propertyName);
 
             if (property != null)
             {
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/DictionaryEqualityComparer.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/DictionaryEqualityComparer.cs
index 40472344b83..8e7a86b1a73 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/DictionaryEqualityComparer.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/DictionaryEqualityComparer.cs
@@ -3,8 +3,6 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Collections
 {
     /// <summary>
@@ -49,9 +47,10 @@ public int GetHashCode(IImmutableDictionary<TKey, TValue> obj)
         {
             int hashCode = 0;
 
-            var concreteDictionary1 = obj as ImmutableDictionary<TKey, TValue>;
+            ImmutableDictionary<TKey, TValue>? concreteDictionary1 = obj as ImmutableDictionary<TKey, TValue>;
             IEqualityComparer<TKey> keyComparer = concreteDictionary1 != null ? concreteDictionary1.KeyComparer : EqualityComparer<TKey>.Default;
             IEqualityComparer<TValue> valueComparer = concreteDictionary1 != null ? concreteDictionary1.ValueComparer : EqualityComparer<TValue>.Default;
+
             if (obj != null)
             {
                 foreach (KeyValuePair<TKey, TValue> pair in obj)
@@ -68,6 +67,7 @@ public int GetHashCode(IImmutableDictionary<TKey, TValue> obj)
         private static bool AreEquivalent(IImmutableDictionary<TKey, TValue> dictionary1, IImmutableDictionary<TKey, TValue> dictionary2)
         {
             Requires.NotNull(dictionary1, "dictionary1");
+
             if (dictionary1 == dictionary2)
             {
                 return true;
@@ -80,7 +80,7 @@ private static bool AreEquivalent(IImmutableDictionary<TKey, TValue> dictionary1
         /// <summary>
         /// Tests two dictionaries to see if their contents are identical.
         /// </summary>
-        private static bool AreEquivalent(IReadOnlyDictionary<TKey, TValue> dictionary1, IReadOnlyDictionary<TKey, TValue> dictionary2, IEqualityComparer<TValue> valueComparer)
+        private static bool AreEquivalent(IReadOnlyDictionary<TKey, TValue>? dictionary1, IReadOnlyDictionary<TKey, TValue>? dictionary2, IEqualityComparer<TValue> valueComparer)
         {
             Requires.NotNull(valueComparer, "valueComparer");
 
@@ -89,7 +89,7 @@ private static bool AreEquivalent(IReadOnlyDictionary<TKey, TValue> dictionary1,
                 return true;
             }
 
-            if ((dictionary1 == null) ^ (dictionary2 == null)) // XOR
+            if (dictionary1 == null || dictionary2 == null)
             {
                 return false;
             }
@@ -116,6 +116,5 @@ private static bool AreEquivalent(IReadOnlyDictionary<TKey, TValue> dictionary1,
 
             return true;
         }
-
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringDictionary.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringDictionary.cs
index fa18fb7db67..a41d8fdc569 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringDictionary.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringDictionary.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace System.Collections.Immutable
 {
     internal static class ImmutableStringDictionary<TValue>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringHashSet.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringHashSet.cs
index b1321afc7f0..e95ac5c6c5a 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringHashSet.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Collections/ImmutableStringHashSet.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace System.Collections.Immutable
 {
     internal static class ImmutableStringHashSet
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Composition/DeconstructionExtensions.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Composition/DeconstructionExtensions.cs
index aaf47edb61c..86e16ef3f49 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Composition/DeconstructionExtensions.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Composition/DeconstructionExtensions.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace System.Collections.Generic
 {
     internal static class DeconstructionExtensions
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/IFileSystem.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/IFileSystem.cs
index c7964f0b52b..38578181401 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/IFileSystem.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/IFileSystem.cs
@@ -4,11 +4,10 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Text;
+
 using Microsoft.VisualStudio.Composition;
 using Microsoft.VisualStudio.ProjectSystem;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.IO
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/SimpleFileWatcher.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/SimpleFileWatcher.cs
index af25b03bd50..9b2f2369493 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/SimpleFileWatcher.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/SimpleFileWatcher.cs
@@ -3,8 +3,6 @@
 using System;
 using System.IO;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.IO
 {
     /// <summary>
@@ -12,9 +10,9 @@ namespace Microsoft.VisualStudio.IO
     /// </summary>
     internal sealed class SimpleFileWatcher : IDisposable
     {
-        private FileSystemWatcher FileWatcher { get; set; }
-        private FileSystemEventHandler _handler;
-        private RenamedEventHandler _renameHandler;
+        private FileSystemWatcher? _fileWatcher;
+        private FileSystemEventHandler? _handler;
+        private RenamedEventHandler? _renameHandler;
 
         // For unit tests
         public SimpleFileWatcher()
@@ -22,9 +20,9 @@ public SimpleFileWatcher()
         }
 
         public SimpleFileWatcher(string dirToWatch, bool includeSubDirs, NotifyFilters notifyFilters, string fileFilter,
-                                       FileSystemEventHandler handler, RenamedEventHandler renameHandler)
+                                 FileSystemEventHandler? handler, RenamedEventHandler? renameHandler)
         {
-            FileWatcher = new FileSystemWatcher(dirToWatch)
+            _fileWatcher = new FileSystemWatcher(dirToWatch)
             {
                 IncludeSubdirectories = includeSubDirs,
                 NotifyFilter = notifyFilters,
@@ -33,42 +31,40 @@ public SimpleFileWatcher(string dirToWatch, bool includeSubDirs, NotifyFilters n
 
             if (handler != null)
             {
-                FileWatcher.Created += handler;
-                FileWatcher.Deleted += handler;
-                FileWatcher.Changed += handler;
+                _fileWatcher.Created += handler;
+                _fileWatcher.Deleted += handler;
+                _fileWatcher.Changed += handler;
             }
 
             if (renameHandler != null)
             {
-                FileWatcher.Renamed += renameHandler;
+                _fileWatcher.Renamed += renameHandler;
             }
-            FileWatcher.EnableRaisingEvents = true;
+
+            _fileWatcher.EnableRaisingEvents = true;
             _handler = handler;
             _renameHandler = renameHandler;
         }
 
-        /// <summary>
-        /// Cleans up our watcher on the Project.Json file
-        /// </summary>
         public void Dispose()
         {
-            if (FileWatcher != null)
+            if (_fileWatcher != null)
             {
-                FileWatcher.EnableRaisingEvents = false;
+                _fileWatcher.EnableRaisingEvents = false;
                 if (_handler != null)
                 {
-                    FileWatcher.Created += _handler;
-                    FileWatcher.Deleted += _handler;
-                    FileWatcher.Changed += _handler;
+                    _fileWatcher.Created -= _handler;
+                    _fileWatcher.Deleted -= _handler;
+                    _fileWatcher.Changed -= _handler;
                     _handler = null;
                 }
                 if (_renameHandler != null)
                 {
-                    FileWatcher.Renamed += _renameHandler;
+                    _fileWatcher.Renamed -= _renameHandler;
                     _renameHandler = null;
                 }
-                FileWatcher.Dispose();
-                FileWatcher = null;
+                _fileWatcher.Dispose();
+                _fileWatcher = null;
             }
         }
     }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/Win32FileSystem.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/Win32FileSystem.cs
index 3697d264d4f..4ac71613769 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/Win32FileSystem.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/IO/Win32FileSystem.cs
@@ -6,8 +6,6 @@
 using System.IO;
 using System.Text;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.IO
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/ObjectPool.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/ObjectPool.cs
index 0bcd498ec36..42ea4d25915 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/ObjectPool.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/ObjectPool.cs
@@ -4,8 +4,6 @@
 using System.Diagnostics;
 using System.Threading;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Buffers.PooledObjects
 {
     /// <summary>
@@ -30,12 +28,12 @@ internal class ObjectPool<T> where T : class
         [DebuggerDisplay("{Value,nq}")]
         private struct Element
         {
-            internal T Value;
+            internal T? Value;
         }
 
         // Storage for the pool objects. The first item is stored in a dedicated field because we
         // expect to be able to satisfy most requests from it.
-        private T _firstItem;
+        private T? _firstItem;
         private readonly Element[] _items;
 
         // factory is stored for the lifetime of the pool. We will call this only when pool needs to
@@ -74,7 +72,7 @@ internal T Allocate()
             // Note that the initial read is optimistically not synchronized. That is intentional. 
             // We will interlock only when we have a candidate. in a worst case we may miss some
             // recently returned objects. Not a big deal.
-            T inst = _firstItem;
+            T? inst = _firstItem;
             if (inst == null || inst != Interlocked.CompareExchange(ref _firstItem, null, inst))
             {
                 inst = AllocateSlow();
@@ -92,7 +90,7 @@ private T AllocateSlow()
                 // Note that the initial read is optimistically not synchronized. That is intentional. 
                 // We will interlock only when we have a candidate. in a worst case we may miss some
                 // recently returned objects. Not a big deal.
-                T inst = items[i].Value;
+                T? inst = items[i].Value;
                 if (inst != null)
                 {
                     if (inst == Interlocked.CompareExchange(ref items[i].Value, null, inst))
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledDictionary.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledDictionary.cs
index 992a9c7d538..708681adf0d 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledDictionary.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledDictionary.cs
@@ -3,8 +3,6 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Buffers.PooledObjects
 {
     // Dictionary that can be recycled via an object pool
@@ -37,8 +35,8 @@ public void Free()
         // if someone needs to create a pool;
         public static ObjectPool<PooledDictionary<K, V>> CreatePool()
         {
-            ObjectPool<PooledDictionary<K, V>> pool = null;
-            pool = new ObjectPool<PooledDictionary<K, V>>(() => new PooledDictionary<K, V>(pool), 128);
+            ObjectPool<PooledDictionary<K, V>>? pool = null;
+            pool = new ObjectPool<PooledDictionary<K, V>>(() => new PooledDictionary<K, V>(pool!), 128);
             return pool;
         }
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledHashSet.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledHashSet.cs
index 42a74de453a..f7e23949c13 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledHashSet.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledHashSet.cs
@@ -2,8 +2,6 @@
 
 using System.Collections.Generic;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Buffers.PooledObjects
 {
     // HashSet that can be recycled via an object pool
@@ -29,8 +27,8 @@ public void Free()
         // if someone needs to create a pool;
         public static ObjectPool<PooledHashSet<T>> CreatePool()
         {
-            ObjectPool<PooledHashSet<T>> pool = null;
-            pool = new ObjectPool<PooledHashSet<T>>(() => new PooledHashSet<T>(pool), 128);
+            ObjectPool<PooledHashSet<T>>? pool = null;
+            pool = new ObjectPool<PooledHashSet<T>>(() => new PooledHashSet<T>(pool!), 128);
             return pool;
         }
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledStringBuilder.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledStringBuilder.cs
index 43f236ae52b..936975250ac 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledStringBuilder.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/PooledObjects/PooledStringBuilder.cs
@@ -3,8 +3,6 @@
 using System;
 using System.Text;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.Buffers.PooledObjects
 {
     /// <summary>
@@ -57,7 +55,6 @@ public string ToStringAndFree(int startIndex, int length)
         // global pool
         private static readonly ObjectPool<PooledStringBuilder> s_poolInstance = CreatePool();
 
-        // if someone needs to create a private pool;
         /// <summary>
         /// If someone need to create a private pool
         /// </summary>
@@ -65,8 +62,8 @@ public string ToStringAndFree(int startIndex, int length)
         /// <returns></returns>
         public static ObjectPool<PooledStringBuilder> CreatePool(int size = 32)
         {
-            ObjectPool<PooledStringBuilder> pool = null;
-            pool = new ObjectPool<PooledStringBuilder>(() => new PooledStringBuilder(pool), size);
+            ObjectPool<PooledStringBuilder>? pool = null;
+            pool = new ObjectPool<PooledStringBuilder>(() => new PooledStringBuilder(pool!), size);
             return pool;
         }
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/ActiveConfiguredProjectsProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/ActiveConfiguredProjectsProvider.cs
index 75214f728e6..76c1867baae 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/ActiveConfiguredProjectsProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/ActiveConfiguredProjectsProvider.cs
@@ -10,8 +10,6 @@
 using Microsoft.VisualStudio.Buffers.PooledObjects;
 using Microsoft.VisualStudio.ProjectSystem.Configuration;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem
 {
     [Export(typeof(IActiveConfiguredProjectsProvider))]
@@ -63,6 +61,9 @@ internal class ActiveConfiguredProjectsProvider : IActiveConfiguredProjectsProvi
         [ImportingConstructor]
         public ActiveConfiguredProjectsProvider(IUnconfiguredProjectServices services, UnconfiguredProject project)
         {
+            Requires.NotNull(services, nameof(services));
+            Requires.NotNull(project, nameof(project));
+
             _services = services;
             _project = project;
 
@@ -75,13 +76,19 @@ public OrderPrecedenceImportCollection<IActiveConfiguredProjectsDimensionProvide
             get;
         }
 
-        public async Task<ImmutableDictionary<string, ConfiguredProject>> GetActiveConfiguredProjectsMapAsync()
+        public async Task<ImmutableDictionary<string, ConfiguredProject>?> GetActiveConfiguredProjectsMapAsync()
         {
-            var builder = PooledDictionary<string, ConfiguredProject>.GetInstance();
+            ActiveConfiguredObjects<ConfiguredProject>? projects = await GetActiveConfiguredProjectsAsync();
 
-            ActiveConfiguredObjects<ConfiguredProject> projects = await GetActiveConfiguredProjectsAsync();
+            if (projects == null || projects.Objects.Count == 0)
+            {
+                return null;
+            }
+
+            var builder = PooledDictionary<string, ConfiguredProject>.GetInstance();
 
             bool isCrossTargeting = projects.Objects.All(project => project.ProjectConfiguration.IsCrossTargeting());
+
             if (isCrossTargeting)
             {
                 foreach (ConfiguredProject project in projects.Objects)
@@ -98,11 +105,14 @@ public async Task<ImmutableDictionary<string, ConfiguredProject>> GetActiveConfi
             return builder.ToImmutableDictionaryAndFree();
         }
 
-        public async Task<ActiveConfiguredObjects<ConfiguredProject>> GetActiveConfiguredProjectsAsync()
+        public async Task<ActiveConfiguredObjects<ConfiguredProject>?> GetActiveConfiguredProjectsAsync()
         {
-            ActiveConfiguredObjects<ProjectConfiguration> configurations = await GetActiveProjectConfigurationsAsync();
+            ActiveConfiguredObjects<ProjectConfiguration>? configurations = await GetActiveProjectConfigurationsAsync();
+
             if (configurations == null)
+            {
                 return null;
+            }
 
             ImmutableArray<ConfiguredProject>.Builder builder = ImmutableArray.CreateBuilder<ConfiguredProject>(configurations.Objects.Count);
 
@@ -116,11 +126,14 @@ public async Task<ActiveConfiguredObjects<ConfiguredProject>> GetActiveConfigure
             return new ActiveConfiguredObjects<ConfiguredProject>(builder.MoveToImmutable(), configurations.DimensionNames);
         }
 
-        public async Task<ActiveConfiguredObjects<ProjectConfiguration>> GetActiveProjectConfigurationsAsync()
+        public async Task<ActiveConfiguredObjects<ProjectConfiguration>?> GetActiveProjectConfigurationsAsync()
         {
             ProjectConfiguration activeSolutionConfiguration = _services.ActiveConfiguredProjectProvider.ActiveProjectConfiguration;
+
             if (activeSolutionConfiguration == null)
+            {
                 return null;
+            }
 
             IImmutableSet<ProjectConfiguration> configurations = await _services.ProjectConfigurationsService.GetKnownProjectConfigurationsAsync();
 
@@ -156,7 +169,9 @@ private static bool IsActiveConfigurationCandidate(ProjectConfiguration activeSo
             foreach ((string dimensionName, string dimensionValue) in activeSolutionConfiguration.Dimensions)
             {
                 if (ignoredDimensionNames.Contains(dimensionName))
+                {
                     continue;
+                }
 
                 if (!configuration.Dimensions.TryGetValue(dimensionName, out string otherDimensionValue) ||
                     !string.Equals(dimensionValue, otherDimensionValue, StringComparison.Ordinal))
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/BuildProperty.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/BuildProperty.cs
index 254d733d7af..aab578c210b 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/BuildProperty.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/BuildProperty.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/CommandLineDesignTimeBuildPropertiesProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/CommandLineDesignTimeBuildPropertiesProvider.cs
index e26fb914fd2..2bcbf4be5de 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/CommandLineDesignTimeBuildPropertiesProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/CommandLineDesignTimeBuildPropertiesProvider.cs
@@ -5,8 +5,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildDesignTimeBuildPropertyProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildDesignTimeBuildPropertyProvider.cs
index 2e730395766..0fef8ea4f5b 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildDesignTimeBuildPropertyProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildDesignTimeBuildPropertyProvider.cs
@@ -5,8 +5,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildPropertyProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildPropertyProvider.cs
index 7a6950f6c7b..a8957c79dde 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildPropertyProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/GeneratePackageOnBuildPropertyProvider.cs
@@ -5,8 +5,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/ImplicitlyActiveConfiguredProjectReadyToBuild.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/ImplicitlyActiveConfiguredProjectReadyToBuild.cs
index ba6099f94d8..52b7db041e5 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/ImplicitlyActiveConfiguredProjectReadyToBuild.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/ImplicitlyActiveConfiguredProjectReadyToBuild.cs
@@ -4,8 +4,6 @@
 using System.ComponentModel.Composition;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     [Export(typeof(IConfiguredProjectReadyToBuild))]
@@ -16,7 +14,7 @@ internal sealed class ImplicitlyActiveConfiguredProjectReadyToBuild : IConfigure
         private readonly ConfiguredProject _configuredProject;
         private readonly IActiveConfiguredProjectProvider _activeConfiguredProjectProvider;
 
-        private TaskCompletionSource<object> _activationTask;
+        private TaskCompletionSource<object?> _activationTask;
 
         [ImportingConstructor]
         public ImplicitlyActiveConfiguredProjectReadyToBuild(
@@ -25,7 +23,7 @@ public ImplicitlyActiveConfiguredProjectReadyToBuild(
         {
             _configuredProject = configuredProject;
             _activeConfiguredProjectProvider = activeConfiguredProjectProvider;
-            _activationTask = new TaskCompletionSource<object>();
+            _activationTask = new TaskCompletionSource<object?>();
 
             _activeConfiguredProjectProvider.Changed += ActiveConfiguredProject_Changed;
         }
@@ -33,6 +31,7 @@ public ImplicitlyActiveConfiguredProjectReadyToBuild(
         private void ActiveConfiguredProject_Changed(object sender, ActiveConfigurationChangedEventArgs e) => GetLatestActivationTask();
 
         public bool IsValidToBuild => GetLatestActivationTask().IsCompleted;
+
         public Task WaitReadyToBuildAsync() => GetLatestActivationTask();
 
         private Task GetLatestActivationTask()
@@ -45,7 +44,7 @@ private Task GetLatestActivationTask()
                 {
                     if (!nowActive)
                     {
-                        _activationTask = new TaskCompletionSource<object>();
+                        _activationTask = new TaskCompletionSource<object?>();
                     }
                 }
                 else if (nowActive)
@@ -65,7 +64,8 @@ public void Dispose()
 
         private bool IsActive()
         {
-            ProjectConfiguration activeConfig = _activeConfiguredProjectProvider.ActiveProjectConfiguration;
+            ProjectConfiguration? activeConfig = _activeConfiguredProjectProvider.ActiveProjectConfiguration;
+
             if (activeConfig == null)
                 return false;
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/PublishableProjectConfigProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/PublishableProjectConfigProvider.cs
index bbc1c719830..f0ad48e8b7f 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/PublishableProjectConfigProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/PublishableProjectConfigProvider.cs
@@ -8,8 +8,6 @@
 
 using Microsoft.VisualStudio.Threading;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TargetFrameworkGlobalBuildPropertyProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TargetFrameworkGlobalBuildPropertyProvider.cs
index ee78b79684b..f362817969c 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TargetFrameworkGlobalBuildPropertyProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TargetFrameworkGlobalBuildPropertyProvider.cs
@@ -5,8 +5,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TurnOffUseHostCompilerIfAvailableBuildPropertiesProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TurnOffUseHostCompilerIfAvailableBuildPropertiesProvider.cs
index 53cb2cea48b..ae4916a8782 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TurnOffUseHostCompilerIfAvailableBuildPropertiesProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Build/TurnOffUseHostCompilerIfAvailableBuildPropertiesProvider.cs
@@ -5,8 +5,6 @@
 using System.Threading;
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Build
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/BaseProjectConfigurationDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/BaseProjectConfigurationDimensionProvider.cs
index 623c29cc7b2..02e1389cece 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/BaseProjectConfigurationDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/BaseProjectConfigurationDimensionProvider.cs
@@ -10,8 +10,6 @@
 using Microsoft.VisualStudio.Buffers.PooledObjects;
 using Microsoft.VisualStudio.Build;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
@@ -26,7 +24,7 @@ internal abstract class BaseProjectConfigurationDimensionProvider : IProjectConf
         /// <param name="dimensionName">Name of the dimension.</param>
         /// <param name="propertyName">Name of the project property containing the dimension values.</param>
         /// <param name="dimensionDefaultValue">The default value of the dimension, for example "AnyCPU".</param>
-        protected BaseProjectConfigurationDimensionProvider(IProjectAccessor projectAccessor, string dimensionName, string propertyName, string dimensionDefaultValue = null)
+        protected BaseProjectConfigurationDimensionProvider(IProjectAccessor projectAccessor, string dimensionName, string propertyName, string? dimensionDefaultValue = null)
         {
             Requires.NotNull(projectAccessor, nameof(projectAccessor));
 
@@ -46,7 +44,7 @@ public string PropertyName
             get;
         }
 
-        public string DimensionDefaultValue
+        public string? DimensionDefaultValue
         {
             get;
         }
@@ -68,8 +66,8 @@ protected virtual async Task<ImmutableArray<string>> GetOrderedPropertyValuesAsy
         {
             Requires.NotNull(project, nameof(project));
 
-            string propertyValue = await GetPropertyValue(project);
-            if (propertyValue == null || string.IsNullOrEmpty(propertyValue))
+            string? propertyValue = await GetPropertyValue(project);
+            if (string.IsNullOrEmpty(propertyValue))
             {
                 return ImmutableArray<string>.Empty;
             }
@@ -144,7 +142,7 @@ public IEnumerable<string> GetBestGuessDimensionNames(ImmutableArray<ProjectProp
 
         public async Task<IEnumerable<KeyValuePair<string, string>>> GetBestGuessDefaultValuesForDimensionsAsync(UnconfiguredProject project)
         {
-            string defaultValue = await FindDefaultValueFromDimensionPropertyAsync(project) ?? DimensionDefaultValue;
+            string? defaultValue = await FindDefaultValueFromDimensionPropertyAsync(project) ?? DimensionDefaultValue;
             if (defaultValue != null)
                 return new[] { new KeyValuePair<string, string>(DimensionName, defaultValue) };
 
@@ -168,7 +166,7 @@ public async Task<IEnumerable<KeyValuePair<string, string>>> GetBestGuessDefault
         /// <remarks>
         /// This needs to get the evaluated property in order to get inherited properties defines in props or targets.
         /// </remarks>
-        protected async Task<string> GetPropertyValue(UnconfiguredProject project, string propertyName = null)
+        protected async Task<string?> GetPropertyValue(UnconfiguredProject project, string? propertyName = null)
         {
             ConfiguredProject configuredProject = await project.GetSuggestedConfiguredProjectAsync();
 
@@ -178,9 +176,9 @@ protected async Task<string> GetPropertyValue(UnconfiguredProject project, strin
             });
         }
 
-        private async Task<string> FindDefaultValueFromDimensionPropertyAsync(UnconfiguredProject project)
+        private async Task<string?> FindDefaultValueFromDimensionPropertyAsync(UnconfiguredProject project)
         {
-            string values = await FindDimensionPropertyAsync(project);
+            string? values = await FindDimensionPropertyAsync(project);
             if (string.IsNullOrEmpty(values))
                 return null;
 
@@ -196,14 +194,14 @@ private async Task<string> FindDefaultValueFromDimensionPropertyAsync(Unconfigur
             return null;
         }
 
-        private Task<string> FindDimensionPropertyAsync(UnconfiguredProject project)
+        private Task<string?> FindDimensionPropertyAsync(UnconfiguredProject project)
         {
             return ProjectAccessor.OpenProjectXmlForReadAsync(
                 project,
                 projectXml => FindDimensionProperty(projectXml)?.GetUnescapedValue());
         }
 
-        private ProjectPropertyElement FindDimensionProperty(ProjectRootElement projectXml)
+        private ProjectPropertyElement? FindDimensionProperty(ProjectRootElement projectXml)
         {
             IEnumerable<ProjectPropertyElement> properties = projectXml.PropertyGroups
                                                                        .SelectMany(group => group.Properties);
@@ -211,7 +209,7 @@ private ProjectPropertyElement FindDimensionProperty(ProjectRootElement projectX
             return FindDimensionProperty(properties);
         }
 
-        private ProjectPropertyElement FindDimensionProperty(IEnumerable<ProjectPropertyElement> properties)
+        private ProjectPropertyElement? FindDimensionProperty(IEnumerable<ProjectPropertyElement> properties)
         {
             // NOTE: We try to somewhat mimic evaluation, but it doesn't have to be exact; its just a guess
             // at what "might" be the default configuration, not what it actually is.
@@ -220,6 +218,5 @@ private ProjectPropertyElement FindDimensionProperty(IEnumerable<ProjectProperty
                                 p => StringComparers.PropertyNames.Equals(PropertyName, p.Name) &&
                                 BuildUtilities.HasWellKnownConditionsThatAlwaysEvaluateToTrue(p));
         }
-
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ConfigurationProjectConfigurationDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ConfigurationProjectConfigurationDimensionProvider.cs
index d89e881379d..83661c9032a 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ConfigurationProjectConfigurationDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ConfigurationProjectConfigurationDimensionProvider.cs
@@ -1,12 +1,11 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.ComponentModel.Composition;
 using System.Threading.Tasks;
 
 using Microsoft.VisualStudio.Build;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
@@ -76,7 +75,9 @@ public override Task OnDimensionValueChangedAsync(ProjectConfigurationDimensionV
         /// <returns>A task for the async operation.</returns>
         private async Task OnConfigurationAddedAsync(UnconfiguredProject project, string configurationName)
         {
-            string evaluatedPropertyValue = await GetPropertyValue(project);
+            string? evaluatedPropertyValue = await GetPropertyValue(project);
+            if (evaluatedPropertyValue == null)
+                throw new InvalidOperationException($"Property {PropertyName} not defined.");
             await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
             {
                 BuildUtilities.AppendPropertyValue(msbuildProject, evaluatedPropertyValue, PropertyName, configurationName);
@@ -91,7 +92,9 @@ await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
         /// <returns>A task for the async operation.</returns>
         private async Task OnConfigurationRemovedAsync(UnconfiguredProject project, string configurationName)
         {
-            string evaluatedPropertyValue = await GetPropertyValue(project);
+            string? evaluatedPropertyValue = await GetPropertyValue(project);
+            if (evaluatedPropertyValue == null)
+                throw new InvalidOperationException($"Property {PropertyName} not defined.");
             await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
             {
                 BuildUtilities.RemovePropertyValue(msbuildProject, evaluatedPropertyValue, PropertyName, configurationName);
@@ -107,7 +110,9 @@ await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
         /// <returns>A task for the async operation.</returns>
         private async Task OnConfigurationRenamedAsync(UnconfiguredProject project, string oldName, string newName)
         {
-            string evaluatedPropertyValue = await GetPropertyValue(project);
+            string? evaluatedPropertyValue = await GetPropertyValue(project);
+            if (evaluatedPropertyValue == null)
+                throw new InvalidOperationException($"Property {PropertyName} not defined.");
             await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
             {
                 BuildUtilities.RenamePropertyValue(msbuildProject, evaluatedPropertyValue, PropertyName, oldName, newName);
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/DimensionProviderOrder.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/DimensionProviderOrder.cs
index 073e0ef150a..56fce0506d1 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/DimensionProviderOrder.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/DimensionProviderOrder.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IActiveConfiguredProjectsDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IActiveConfiguredProjectsDimensionProvider.cs
index 0a4b569efff..7be4cb1c7b4 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IActiveConfiguredProjectsDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IActiveConfiguredProjectsDimensionProvider.cs
@@ -2,8 +2,6 @@
 
 using Microsoft.VisualStudio.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IConfigurationDimensionDescriptionMetadataView.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IConfigurationDimensionDescriptionMetadataView.cs
index b58843c5098..e705d4312e3 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IConfigurationDimensionDescriptionMetadataView.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IConfigurationDimensionDescriptionMetadataView.cs
@@ -1,8 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IImplicitlyActiveDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IImplicitlyActiveDimensionProvider.cs
index 2b05c34b413..edb125c996a 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IImplicitlyActiveDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IImplicitlyActiveDimensionProvider.cs
@@ -4,8 +4,6 @@
 using System.Collections.Generic;
 using Microsoft.VisualStudio.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IProjectConfigurationDimensionsProvider4.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IProjectConfigurationDimensionsProvider4.cs
index 6b152c032b9..706d1d27bb3 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IProjectConfigurationDimensionsProvider4.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/IProjectConfigurationDimensionsProvider4.cs
@@ -1,10 +1,10 @@
-using System.Collections.Generic;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System.Collections.Generic;
 using System.Collections.Immutable;
 
 using Microsoft.Build.Construction;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem
 {
     internal interface IProjectConfigurationDimensionsProvider4 : IProjectConfigurationDimensionsProvider3
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ImplicitlyActiveDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ImplicitlyActiveDimensionProvider.cs
index a47dce0aac3..8664e5bc6c2 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ImplicitlyActiveDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/ImplicitlyActiveDimensionProvider.cs
@@ -8,8 +8,6 @@
 
 using Microsoft.VisualStudio.Buffers.PooledObjects;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/PlatformProjectConfigurationDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/PlatformProjectConfigurationDimensionProvider.cs
index 9c006e3d9eb..e651de70f1f 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/PlatformProjectConfigurationDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/PlatformProjectConfigurationDimensionProvider.cs
@@ -1,12 +1,11 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.ComponentModel.Composition;
 using System.Threading.Tasks;
 
 using Microsoft.VisualStudio.Build;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
@@ -67,7 +66,9 @@ public override Task OnDimensionValueChangedAsync(ProjectConfigurationDimensionV
         /// <returns>A task for the async operation.</returns>
         private async Task OnPlatformAddedAsync(UnconfiguredProject project, string platformName)
         {
-            string evaluatedPropertyValue = await GetPropertyValue(project);
+            string? evaluatedPropertyValue = await GetPropertyValue(project);
+            if (evaluatedPropertyValue == null)
+                throw new InvalidOperationException($"Property {PropertyName} not defined.");
             await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
             {
                 BuildUtilities.AppendPropertyValue(msbuildProject, evaluatedPropertyValue, PropertyName, platformName);
@@ -82,7 +83,9 @@ await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
         /// <returns>A task for the async operation.</returns>
         private async Task OnPlatformDeletedAsync(UnconfiguredProject project, string platformName)
         {
-            string evaluatedPropertyValue = await GetPropertyValue(project);
+            string? evaluatedPropertyValue = await GetPropertyValue(project);
+            if (evaluatedPropertyValue == null)
+                throw new InvalidOperationException($"Property {PropertyName} not defined.");
             await ProjectAccessor.OpenProjectXmlForWriteAsync(project, msbuildProject =>
             {
                 BuildUtilities.RemovePropertyValue(msbuildProject, evaluatedPropertyValue, PropertyName, platformName);
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/TargetFrameworkProjectConfigurationDimensionProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/TargetFrameworkProjectConfigurationDimensionProvider.cs
index 8203ab26842..cd188f845dd 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/TargetFrameworkProjectConfigurationDimensionProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Configuration/TargetFrameworkProjectConfigurationDimensionProvider.cs
@@ -6,8 +6,6 @@
 
 using Microsoft.VisualStudio.Build;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Configuration
 {
     /// <summary>
@@ -36,7 +34,7 @@ protected override async Task<ImmutableArray<string>> GetOrderedPropertyValuesAs
         {
             Requires.NotNull(project, nameof(project));
 
-            string targetFrameworksProperty = await GetPropertyValue(project, ConfigurationGeneral.TargetFrameworksProperty);
+            string? targetFrameworksProperty = await GetPropertyValue(project, ConfigurationGeneral.TargetFrameworksProperty);
             if (targetFrameworksProperty != null)
             {
                 return BuildUtilities.GetPropertyValues(targetFrameworksProperty).ToImmutableArray();
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/ActiveDebugFrameworkServices.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/ActiveDebugFrameworkServices.cs
index e6fbcc4fd0f..83619c546f5 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/ActiveDebugFrameworkServices.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/ActiveDebugFrameworkServices.cs
@@ -8,8 +8,6 @@
 
 using Microsoft.VisualStudio.Build;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Debug
 {
     /// <summary>
@@ -32,7 +30,7 @@ public ActiveDebugFrameworkServices(IActiveConfiguredProjectsProvider activeConf
         /// <summary>
         /// <see cref="IActiveDebugFrameworkServices.GetProjectFrameworksAsync"/>
         /// </summary>
-        public async Task<List<string>> GetProjectFrameworksAsync()
+        public async Task<List<string>?> GetProjectFrameworksAsync()
         {
             // It is important that we return the frameworks in the order they are specified in the project to ensure the default is set
             // correctly. 
@@ -40,11 +38,12 @@ public async Task<List<string>> GetProjectFrameworksAsync()
 
             string targetFrameworks = (string)await props.TargetFrameworks.GetValueAsync();
 
-            if (!string.IsNullOrWhiteSpace(targetFrameworks))
+            if (string.IsNullOrWhiteSpace(targetFrameworks))
             {
-                return BuildUtilities.GetPropertyValues(targetFrameworks).ToList();
+                return null;
             }
-            return null;
+
+            return BuildUtilities.GetPropertyValues(targetFrameworks).ToList();
         }
 
         /// <summary>
@@ -59,29 +58,35 @@ public async Task SetActiveDebuggingFrameworkPropertyAsync(string activeFramewor
         /// <summary>
         /// <see cref="IActiveDebugFrameworkServices.GetActiveDebuggingFrameworkPropertyAsync"/>
         /// </summary>
-        public async Task<string> GetActiveDebuggingFrameworkPropertyAsync()
+        public async Task<string?> GetActiveDebuggingFrameworkPropertyAsync()
         {
             ProjectDebugger props = await _commonProjectServices.ActiveConfiguredProjectProperties.GetProjectDebuggerPropertiesAsync();
-            string activeValue = await props.ActiveDebugFramework.GetValueAsync() as string;
+            string? activeValue = await props.ActiveDebugFramework.GetValueAsync() as string;
             return activeValue;
         }
 
         /// <summary>
         /// <see cref="IActiveDebugFrameworkServices.GetConfiguredProjectForActiveFrameworkAsync"/>
         /// </summary>
-        public async Task<ConfiguredProject> GetConfiguredProjectForActiveFrameworkAsync()
+        public async Task<ConfiguredProject?> GetConfiguredProjectForActiveFrameworkAsync()
         {
 #pragma warning disable CS0618 // Type or member is obsolete
-            ImmutableDictionary<string, ConfiguredProject> configProjects = await _activeConfiguredProjectsProvider.GetActiveConfiguredProjectsMapAsync();
+            ImmutableDictionary<string, ConfiguredProject>? configProjects = await _activeConfiguredProjectsProvider.GetActiveConfiguredProjectsMapAsync();
 #pragma warning restore CS0618 // Type or member is obsolete
 
+            if (configProjects == null)
+            {
+                return null;
+            }
+
             // If there is only one we are done
             if (configProjects.Count == 1)
             {
                 return configProjects.First().Value;
             }
 
-            string activeFramework = await GetActiveDebuggingFrameworkPropertyAsync();
+            string? activeFramework = await GetActiveDebuggingFrameworkPropertyAsync();
+
             if (!string.IsNullOrWhiteSpace(activeFramework))
             {
                 if (configProjects.TryGetValue(activeFramework, out ConfiguredProject configuredProject))
@@ -92,7 +97,8 @@ public async Task<ConfiguredProject> GetConfiguredProjectForActiveFrameworkAsync
 
             // We can't just select the first one. If activeFramework is not set we must pick the first one as defined by the 
             // targetFrameworks property. So we need the order as returned by GetProjectFrameworks()
-            List<string> frameworks = await GetProjectFrameworksAsync();
+            List<string>? frameworks = await GetProjectFrameworksAsync();
+
             if (frameworks != null && frameworks.Count > 0)
             {
                 if (configProjects.TryGetValue(frameworks[0], out ConfiguredProject configuredProject))
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/IActiveDebugFrameworkServices.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/IActiveDebugFrameworkServices.cs
index 015c5507d73..01bf2b9e6ca 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/IActiveDebugFrameworkServices.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/IActiveDebugFrameworkServices.cs
@@ -2,15 +2,14 @@
 
 using System.Collections.Generic;
 using System.Threading.Tasks;
-using Microsoft.VisualStudio.Composition;
 
-#nullable disable
+using Microsoft.VisualStudio.Composition;
 
 namespace Microsoft.VisualStudio.ProjectSystem.Debug
 {
     /// <summary>
     /// Wrapper around the active debug framework to provide a single implementation of what is considered the active framework. If there is
-    /// only one framework
+    /// only one framework.
     /// </summary>
     [ProjectSystemContract(ProjectSystemContractScope.UnconfiguredProject, ProjectSystemContractProvider.Private, Cardinality = ImportCardinality.ExactlyOne)]
     public interface IActiveDebugFrameworkServices
@@ -18,21 +17,21 @@ public interface IActiveDebugFrameworkServices
         /// <summary>
         /// Returns the set of frameworks in the order defined in msbuild. If not multi-targeting it returns null.
         /// </summary>
-        Task<List<string>> GetProjectFrameworksAsync();
+        Task<List<string>?> GetProjectFrameworksAsync();
 
         /// <summary>
-        /// Sets the value of the active debugging target framework property
+        /// Sets the value of the active debugging target framework property.
         /// </summary>
         Task SetActiveDebuggingFrameworkPropertyAsync(string activeFramework);
 
         /// <summary>
-        /// Returns the value of the property, or empty string/null if the property has never been set
+        /// Returns the value of the property, or empty string/null if the property has never been set.
         /// </summary>
-        Task<string> GetActiveDebuggingFrameworkPropertyAsync();
+        Task<string?> GetActiveDebuggingFrameworkPropertyAsync();
 
         /// <summary>
-        /// Returns the configured project which represents the active framework. This is valid whether multi-targeting or not
+        /// Returns the configured project which represents the active framework. This is valid whether multi-targeting or not.
         /// </summary>
-        Task<ConfiguredProject> GetConfiguredProjectForActiveFrameworkAsync();
+        Task<ConfiguredProject?> GetConfiguredProjectForActiveFrameworkAsync();
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/IProjectExportProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/IProjectExportProvider.cs
index d869b375b3e..64001fdc58a 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/IProjectExportProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/IProjectExportProvider.cs
@@ -2,8 +2,6 @@
 
 using Microsoft.VisualStudio.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Extensibility
 {
     /// <summary>
@@ -16,8 +14,8 @@ public interface IProjectExportProvider
         /// <summary>
         /// Returns the export for the given project without having to go to the 
         /// UI thread. This is the preferred method for getting access to project specific
-        /// exports
+        /// exports.
         /// </summary>
-        T GetExport<T>(string projectFilePath) where T : class;
+        T? GetExport<T>(string projectFilePath) where T : class;
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/ProjectExportProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/ProjectExportProvider.cs
index 133e89f9f7f..169ab7a9ff7 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/ProjectExportProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Extensibility/ProjectExportProvider.cs
@@ -4,8 +4,6 @@
 using System.ComponentModel.Composition;
 using System.Linq;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Extensibility
 {
     /// <summary>
@@ -28,7 +26,7 @@ public ProjectExportProvider(IProjectServiceAccessor serviceAccessor)
         /// UI thread. This is the preferred method for getting access to project specific
         /// exports
         /// </summary>
-        public T GetExport<T>(string projectFilePath) where T : class
+        public T? GetExport<T>(string projectFilePath) where T : class
         {
             if (string.IsNullOrEmpty(projectFilePath))
             {
@@ -37,7 +35,7 @@ public T GetExport<T>(string projectFilePath) where T : class
 
             IProjectService projectService = _projectServiceAccessor.GetProjectService();
 
-            UnconfiguredProject project = projectService?.LoadedUnconfiguredProjects
+            UnconfiguredProject? project = projectService?.LoadedUnconfiguredProjects
                 .FirstOrDefault(x => x.FullPath.Equals(projectFilePath, StringComparison.OrdinalIgnoreCase));
 
             return project?.Services.ExportProvider.GetExportedValueOrDefault<T>();
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IActiveConfiguredProjectsProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IActiveConfiguredProjectsProvider.cs
index 0bc78ea8bdb..bc5f1213583 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IActiveConfiguredProjectsProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IActiveConfiguredProjectsProvider.cs
@@ -3,9 +3,8 @@
 using System;
 using System.Collections.Immutable;
 using System.Threading.Tasks;
-using Microsoft.VisualStudio.Composition;
 
-#nullable disable
+using Microsoft.VisualStudio.Composition;
 
 namespace Microsoft.VisualStudio.ProjectSystem
 {
@@ -22,12 +21,16 @@ namespace Microsoft.VisualStudio.ProjectSystem
     internal interface IActiveConfiguredProjectsProvider
     {
         /// <summary>
-        /// Gets all the active configured projects by TargetFramework dimension for the current unconfigured project.
-        /// If the current project is not a cross-targeting project, then it returns a singleton key-value pair with an ignorable key and single active configured project as value.
+        ///     Gets all the active configured projects by TargetFramework dimension for the current unconfigured project.
+        ///     If the current project is not a cross-targeting project, then it returns a singleton key-value pair with an
+        ///     ignorable key and single active configured project as value.
         /// </summary>
-        /// <returns>Map from TargetFramework dimension to active configured project.</returns>
+        /// <returns>
+        ///     Map from TargetFramework dimension to active configured project, or <see langword="null" /> if there
+        ///     are no active <see cref="ConfiguredProject"/> objects.
+        /// </returns>
         [Obsolete("This method will be removed in a future build.")]
-        Task<ImmutableDictionary<string, ConfiguredProject>> GetActiveConfiguredProjectsMapAsync();
+        Task<ImmutableDictionary<string, ConfiguredProject>?> GetActiveConfiguredProjectsMapAsync();
 
         /// <summary>
         ///     Returns the ordered list of configured projects that are active for the current project, loading them if needed.
@@ -42,7 +45,7 @@ internal interface IActiveConfiguredProjectsProvider
         ///     The order in the returned <see cref="ActiveConfiguredObjects{T}"/> matches the declared ordered within
         ///     the project file.
         /// </remarks>
-        Task<ActiveConfiguredObjects<ConfiguredProject>> GetActiveConfiguredProjectsAsync();
+        Task<ActiveConfiguredObjects<ConfiguredProject>?> GetActiveConfiguredProjectsAsync();
 
         /// <summary>
         ///     Returns the ordered list of project configurations that are active for the current project.
@@ -57,6 +60,6 @@ internal interface IActiveConfiguredProjectsProvider
         ///     The order in the returned <see cref="ActiveConfiguredObjects{T}"/> matches the declared ordered within
         ///     the project file.
         /// </remarks>
-        Task<ActiveConfiguredObjects<ProjectConfiguration>> GetActiveProjectConfigurationsAsync();
+        Task<ActiveConfiguredObjects<ProjectConfiguration>?> GetActiveProjectConfigurationsAsync();
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IProjectAccessor.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IProjectAccessor.cs
index d7fabdb3b96..5502f743942 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IProjectAccessor.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/IProjectAccessor.cs
@@ -8,8 +8,6 @@
 using Microsoft.Build.Evaluation;
 using Microsoft.VisualStudio.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/IProjectImageProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/IProjectImageProvider.cs
index b3de87f5376..84cc51ab3f1 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/IProjectImageProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/IProjectImageProvider.cs
@@ -2,8 +2,6 @@
 
 using Microsoft.VisualStudio.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Imaging
 {
     /// <summary>
@@ -16,6 +14,6 @@ internal interface IProjectImageProvider
         ///     Returns the <see cref="ProjectImageMoniker"/> for the specified key, returning <see langword="null"/>
         ///     if the provider does handle the specified key.
         /// </summary>
-        ProjectImageMoniker GetProjectImage(string key);
+        ProjectImageMoniker? GetProjectImage(string key);
     }
 }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageKey.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageKey.cs
index b511c55fdc3..882c2da0b14 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageKey.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageKey.cs
@@ -1,7 +1,5 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Imaging
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageProviderAggregator.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageProviderAggregator.cs
index 81236f00f52..d88d11e9301 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageProviderAggregator.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Imaging/ProjectImageProviderAggregator.cs
@@ -2,9 +2,8 @@
 
 using System;
 using System.ComponentModel.Composition;
-using Microsoft.VisualStudio.Composition;
 
-#nullable disable
+using Microsoft.VisualStudio.Composition;
 
 namespace Microsoft.VisualStudio.ProjectSystem.Imaging
 {
@@ -29,16 +28,18 @@ public OrderPrecedenceImportCollection<IProjectImageProvider> ImageProviders
             get;
         }
 
-        public ProjectImageMoniker GetProjectImage(string key)
+        public ProjectImageMoniker? GetProjectImage(string key)
         {
             Requires.NotNullOrEmpty(key, nameof(key));
 
             foreach (Lazy<IProjectImageProvider> provider in ImageProviders)
             {
-                ProjectImageMoniker image = provider.Value.GetProjectImage(key);
+                ProjectImageMoniker? image = provider.Value.GetProjectImage(key);
+
                 if (image != null)
+                {
                     return image;
-
+                }
             }
 
             return null;
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractProjectCommand.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractProjectCommand.cs
index a074c8c2ed8..fcbb6522824 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractProjectCommand.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractProjectCommand.cs
@@ -6,8 +6,6 @@
 
 using Microsoft.VisualStudio.Threading;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Input
 {
     /// <summary>
@@ -22,7 +20,7 @@ protected AbstractProjectCommand()
             _commandIds = new Lazy<long[]>(() => GetCommandIds(this));
         }
 
-        public Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, long commandId, bool focused, string commandText, CommandStatus progressiveStatus)
+        public Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, long commandId, bool focused, string? commandText, CommandStatus progressiveStatus)
         {
             Requires.NotNull(nodes, nameof(nodes));
 
@@ -48,7 +46,7 @@ public Task<bool> TryHandleCommandAsync(IImmutableSet<IProjectTree> nodes, long
             return TaskResult.False;
         }
 
-        protected abstract Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, bool focused, string commandText, CommandStatus progressiveStatus);
+        protected abstract Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, bool focused, string? commandText, CommandStatus progressiveStatus);
 
         protected abstract Task<bool> TryHandleCommandAsync(IImmutableSet<IProjectTree> nodes, bool focused, long commandExecuteOptions, IntPtr variantArgIn, IntPtr variantArgOut);
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractSingleNodeProjectCommand.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractSingleNodeProjectCommand.cs
index 66dc5629bf6..a9a1fce6756 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractSingleNodeProjectCommand.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/AbstractSingleNodeProjectCommand.cs
@@ -7,8 +7,6 @@
 
 using Microsoft.VisualStudio.Threading;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Input
 {
     /// <summary>
@@ -20,7 +18,7 @@ protected AbstractSingleNodeProjectCommand()
         {
         }
 
-        protected sealed override Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, bool focused, string commandText, CommandStatus progressiveStatus)
+        protected sealed override Task<CommandStatusResult> GetCommandStatusAsync(IImmutableSet<IProjectTree> nodes, bool focused, string? commandText, CommandStatus progressiveStatus)
         {
             if (nodes.Count == 1)
             {
@@ -40,7 +38,7 @@ protected sealed override Task<bool> TryHandleCommandAsync(IImmutableSet<IProjec
             return TaskResult.False;
         }
 
-        protected abstract Task<CommandStatusResult> GetCommandStatusAsync(IProjectTree node, bool focused, string commandText, CommandStatus progressiveStatus);
+        protected abstract Task<CommandStatusResult> GetCommandStatusAsync(IProjectTree node, bool focused, string? commandText, CommandStatus progressiveStatus);
 
         protected abstract Task<bool> TryHandleCommandAsync(IProjectTree node, bool focused, long commandExecuteOptions, IntPtr variantArgIn, IntPtr variantArgOut);
     }
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/GetCommandStatusResult.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/GetCommandStatusResult.cs
index 177d6d8b70e..805b9f50f63 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/GetCommandStatusResult.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/GetCommandStatusResult.cs
@@ -2,8 +2,6 @@
 
 using System.Threading.Tasks;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Input
 {
     internal static class GetCommandStatusResult
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/ProjectCommandAttribute.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/ProjectCommandAttribute.cs
index ddbf5ad55d6..4c78ec09c99 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/ProjectCommandAttribute.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Input/ProjectCommandAttribute.cs
@@ -3,8 +3,6 @@
 using System;
 using System.ComponentModel.Composition;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.Input
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Resources/ManagedImageMonikers.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Resources/ManagedImageMonikers.cs
index f7cb2e42076..cde81b7e336 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Resources/ManagedImageMonikers.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Resources/ManagedImageMonikers.cs
@@ -10,8 +10,6 @@
 [assembly: DebuggerDisplay("{Microsoft.VisualStudio.ProjectSystem.VS.ManagedImageMonikers.ImageMonikerDebugDisplay(this)}", Target = typeof(ImageMoniker))]
 [assembly: DebuggerDisplay("{Microsoft.VisualStudio.ProjectSystem.VS.ManagedImageMonikers.ProjectImageMonikerDebugDisplay(this)}", Target = typeof(ProjectImageMoniker))]
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS
 {
     /// <summary>
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/CrossTarget/AggregateCrossTargetProjectContextProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/CrossTarget/AggregateCrossTargetProjectContextProvider.cs
index 4032782a533..e934c523aca 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/CrossTarget/AggregateCrossTargetProjectContextProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/CrossTarget/AggregateCrossTargetProjectContextProvider.cs
@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.ComponentModel.Composition;
@@ -35,8 +36,14 @@ public async Task<AggregateCrossTargetProjectContext> CreateProjectContextAsync(
         {
             // Get the set of active configured projects ignoring target framework.
 #pragma warning disable CS0618 // Type or member is obsolete
-            ImmutableDictionary<string, ConfiguredProject> configuredProjectsMap = await _activeConfiguredProjectsProvider.GetActiveConfiguredProjectsMapAsync();
+            ImmutableDictionary<string, ConfiguredProject>? configuredProjectsMap = await _activeConfiguredProjectsProvider.GetActiveConfiguredProjectsMapAsync();
 #pragma warning restore CS0618 // Type or member is obsolete
+
+            if (configuredProjectsMap == null)
+            {
+                throw new InvalidOperationException("There are no active configured projects.");
+            }
+
             ProjectConfiguration activeProjectConfiguration = _commonServices.ActiveConfiguredProject.ProjectConfiguration;
             ImmutableArray<ITargetFramework>.Builder targetFrameworks = ImmutableArray.CreateBuilder<ITargetFramework>(initialCapacity: configuredProjectsMap.Count);
             ITargetFramework activeTargetFramework = TargetFramework.Empty;
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/Snapshot/AggregateDependenciesSnapshotProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/Snapshot/AggregateDependenciesSnapshotProvider.cs
index 8b20988d146..2092d2d370e 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/Snapshot/AggregateDependenciesSnapshotProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Dependencies/Snapshot/AggregateDependenciesSnapshotProvider.cs
@@ -93,7 +93,7 @@ void OnSnapshotChanged(object sender, SnapshotChangedEventArgs e)
 
             lock (_snapshotProviders)
             {
-                if (!_snapshotProviders.TryGetValue(projectFilePath, out IDependenciesSnapshotProvider snapshotProvider))
+                if (!_snapshotProviders.TryGetValue(projectFilePath, out IDependenciesSnapshotProvider? snapshotProvider))
                 {
                     snapshotProvider = _projectExportProvider.GetExport<IDependenciesSnapshotProvider>(projectFilePath);
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/AppDesignerFolderProjectImageProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/AppDesignerFolderProjectImageProvider.cs
index 1993119c61a..4f41d2709ec 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/AppDesignerFolderProjectImageProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/AppDesignerFolderProjectImageProvider.cs
@@ -5,8 +5,6 @@
 using Microsoft.VisualStudio.Imaging;
 using Microsoft.VisualStudio.ProjectSystem.Imaging;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Imaging
 {
     /// <summary>
@@ -21,7 +19,7 @@ public AppDesignerFolderProjectImageProvider()
         {
         }
 
-        public ProjectImageMoniker GetProjectImage(string key)
+        public ProjectImageMoniker? GetProjectImage(string key)
         {
             Requires.NotNullOrEmpty(key, nameof(key));
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/CSharp/CSharpProjectImageProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/CSharp/CSharpProjectImageProvider.cs
index 3bf707e833a..f7c9483c28b 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/CSharp/CSharpProjectImageProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/CSharp/CSharpProjectImageProvider.cs
@@ -5,8 +5,6 @@
 using Microsoft.VisualStudio.Imaging;
 using Microsoft.VisualStudio.ProjectSystem.Imaging;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Imaging.CSharp
 {
     /// <summary>
@@ -21,7 +19,7 @@ public CSharpProjectImageProvider()
         {
         }
 
-        public ProjectImageMoniker GetProjectImage(string key)
+        public ProjectImageMoniker? GetProjectImage(string key)
         {
             Requires.NotNullOrEmpty(key, nameof(key));
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/FSharp/FSharpProjectImageProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/FSharp/FSharpProjectImageProvider.cs
index 842d2093463..a9d5b3d6906 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/FSharp/FSharpProjectImageProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/FSharp/FSharpProjectImageProvider.cs
@@ -5,8 +5,6 @@
 using Microsoft.VisualStudio.Imaging;
 using Microsoft.VisualStudio.ProjectSystem.Imaging;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Imaging.FSharp
 {
     /// <summary>
@@ -21,7 +19,7 @@ public FSharpProjectImageProvider()
         {
         }
 
-        public ProjectImageMoniker GetProjectImage(string key)
+        public ProjectImageMoniker? GetProjectImage(string key)
         {
             Requires.NotNullOrEmpty(key, nameof(key));
 
diff --git a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/VisualBasic/VisualBasicProjectImageProvider.cs b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/VisualBasic/VisualBasicProjectImageProvider.cs
index 2614f90b22b..62312ea6e48 100644
--- a/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/VisualBasic/VisualBasicProjectImageProvider.cs
+++ b/src/Microsoft.VisualStudio.ProjectSystem.Managed/Tree/Imaging/VisualBasic/VisualBasicProjectImageProvider.cs
@@ -5,8 +5,6 @@
 using Microsoft.VisualStudio.Imaging;
 using Microsoft.VisualStudio.ProjectSystem.Imaging;
 
-#nullable disable
-
 namespace Microsoft.VisualStudio.ProjectSystem.VS.Imaging.VisualBasic
 {
     /// <summary>
@@ -21,7 +19,7 @@ public VisualBasicProjectImageProvider()
         {
         }
 
-        public ProjectImageMoniker GetProjectImage(string key)
+        public ProjectImageMoniker? GetProjectImage(string key)
         {
             Requires.NotNullOrEmpty(key, nameof(key));