Update based on the following plan:

1. Mark DiagnosticSource.Write(string,object) as RequiresUnreferencedCode
2. Suppress the warnings for any .NET libraries that call the DiagnosticSource.Write() API, and annotate the .NET types being passed in to preserve their important properties.
    - This was done for HttpClient. ASP.NET and EF will need separate changes when those assemblies are made trim compatible
3. Annotate Activity and its small closure of types (ActivityLink, ActivityEvent, ActivityContext, etc) to ensure none of those properties are trimmed.
4. Suppress trim warnings inside DiagnosticSourceEventSource since the public Write method is marked with RequiresUnreferencedCode.

src/libraries/System.Diagnostics.DiagnosticSource/src/System.Diagnostics.DiagnosticSource.csproj

@@ -29,6 +29,9 @@
     <None Include="DiagnosticSourceUsersGuide.md" />
   </ItemGroup>
   <ItemGroup Condition="$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)')) != '.NETCoreApp'">
+    <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\DynamicDependencyAttribute.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\DynamicallyAccessedMemberTypes.cs" />
+    <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\RequiresUnreferencedCodeAttribute.cs" />
     <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\UnconditionalSuppressMessageAttribute.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFramework)' != 'netstandard1.1'">

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticListener.cs

@@ -3,6 +3,7 @@
 
 using System.Threading;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Diagnostics
 {
@@ -255,6 +256,7 @@ public override bool IsEnabled(string name, object? arg1, object? arg2 = null)
         /// <summary>
         /// Override abstract method
         /// </summary>
+        [RequiresUnreferencedCode(WriteRequiresUnreferencedCode)]
         public override void Write(string name, object? value)
         {
             for (DiagnosticSubscription? curSubscription = _subscriptions; curSubscription != null; curSubscription = curSubscription.Next)

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticSource.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Diagnostics
 {
@@ -15,6 +16,8 @@ namespace System.Diagnostics
     /// </summary>
     public abstract partial class DiagnosticSource
     {
+        internal const string WriteRequiresUnreferencedCode = "The type of object being written to DiagnosticSource cannot be discovered statically.";
+
         /// <summary>
         /// Write is a generic way of logging complex payloads.  Each notification
         /// is given a name, which identifies it as well as a object (typically an anonymous type)
@@ -31,6 +34,7 @@ public abstract partial class DiagnosticSource
         /// <param name="name">The name of the event being written.</param>
         /// <param name="value">An object that represent the value being passed as a payload for the event.
         /// This is often an anonymous type which contains several sub-values.</param>
+        [RequiresUnreferencedCode(WriteRequiresUnreferencedCode)]
         public abstract void Write(string name, object? value);
 
         /// <summary>

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticSourceActivity.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Diagnostics
 {
@@ -24,6 +25,7 @@ public abstract partial class DiagnosticSource
         /// <param name="args">An object that represent the value being passed as a payload for the event.</param>
         /// <returns>Started Activity for convenient chaining</returns>
         /// <seealso cref="Activity"/>
+        [RequiresUnreferencedCode(WriteRequiresUnreferencedCode)]
         public Activity StartActivity(Activity activity, object? args)
         {
             activity.Start();
@@ -41,6 +43,7 @@ public Activity StartActivity(Activity activity, object? args)
         /// <param name="activity">Activity to be stopped</param>
         /// <param name="args">An object that represent the value being passed as a payload for the event.</param>
         /// <seealso cref="Activity"/>
+        [RequiresUnreferencedCode(WriteRequiresUnreferencedCode)]
         public void StopActivity(Activity activity, object? args)
         {
             // Stop sets the end time if it was unset, but we want it set before we issue the write

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/DiagnosticSourceEventSource.cs

@@ -717,7 +717,9 @@ public FilterAndTransform(string filterAndPayloadSpec, int startIdx, int endIdx,
                         if (eventNameFilter != null)
                             eventNameFilterPredicate = (string eventName) => eventNameFilter == eventName;
 
-                        var subscription = newListener.Subscribe(new CallbackObserver<KeyValuePair<string, object?>>(delegate (KeyValuePair<string, object?> evnt)
+                        [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                            Justification = "DiagnosticSource.Write is marked with RequiresUnreferencedCode.")]
+                        void OnEventWritten(KeyValuePair<string, object?> evnt)
                         {
                             // The filter given to the DiagnosticSource may not work if users don't is 'IsEnabled' as expected.
                             // Thus we look for any events that may have snuck through and filter them out before forwarding.
@@ -727,7 +729,9 @@ public FilterAndTransform(string filterAndPayloadSpec, int startIdx, int endIdx,
                             var outputArgs = this.Morph(evnt.Value);
                             var eventName = evnt.Key;
                             writeEvent(newListener.Name, eventName, outputArgs);
-                        }), eventNameFilterPredicate);
+                        }
+
+                        var subscription = newListener.Subscribe(new CallbackObserver<KeyValuePair<string, object?>>(OnEventWritten), eventNameFilterPredicate);
                         _liveSubscriptions = new Subscriptions(subscription, _liveSubscriptions);
                     }
                 }));
@@ -948,41 +952,55 @@ internal static void CreateActivityListener(DiagnosticSourceEventSource eventSou
                     return false;
                 };
 
-                eventSource._activityListener.ActivityStarted = activity =>
+                eventSource._activityListener.ActivityStarted = activity => OnActivityStarted(eventSource, activity);
+
+                eventSource._activityListener.ActivityStopped = activity => OnActivityStopped(eventSource, activity);
+
+                ActivitySource.AddActivityListener(eventSource._activityListener);
+            }
+
+            [DynamicDependency(DynamicallyAccessedMemberTypes.PublicProperties, typeof(Activity))]
+            [DynamicDependency(DynamicallyAccessedMemberTypes.PublicProperties, typeof(ActivityContext))]
+            [DynamicDependency(DynamicallyAccessedMemberTypes.PublicProperties, typeof(ActivityEvent))]
+            [DynamicDependency(DynamicallyAccessedMemberTypes.PublicProperties, typeof(ActivityLink))]
+            [DynamicDependency(nameof(DateTime.Ticks), typeof(DateTime))]
+            [DynamicDependency(nameof(TimeSpan.Ticks), typeof(TimeSpan))]
+            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                Justification = "Activity's properties are being preserved with the DynamicDependencies on OnActivityStarted.")]
+            private static void OnActivityStarted(DiagnosticSourceEventSource eventSource, Activity activity)
+            {
+                FilterAndTransform? list = eventSource._activitySourceSpecs;
+                while (list != null)
                 {
-                    FilterAndTransform? list = eventSource._activitySourceSpecs;
-                    while (list != null)
+                    if ((list.Events & ActivityEvents.ActivityStart) != 0 &&
+                        (activity.Source.Name == list.SourceName || list.SourceName == "*") &&
+                        (list.ActivityName == null || list.ActivityName == activity.OperationName))
                     {
-                        if ((list.Events & ActivityEvents.ActivityStart) != 0 &&
-                            (activity.Source.Name == list.SourceName || list.SourceName == "*") &&
-                            (list.ActivityName == null || list.ActivityName == activity.OperationName))
-                        {
-                            eventSource.ActivityStart(activity.Source.Name, activity.OperationName, list.Morph(activity));
-                            return;
-                        }
-
-                        list = list.Next;
+                        eventSource.ActivityStart(activity.Source.Name, activity.OperationName, list.Morph(activity));
+                        return;
                     }
-                };
 
-                eventSource._activityListener.ActivityStopped = activity =>
+                    list = list.Next;
+                }
+            }
+
+            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                Justification = "Activity's properties are being preserved with the DynamicDependencies on OnActivityStarted.")]
+            private static void OnActivityStopped(DiagnosticSourceEventSource eventSource, Activity activity)
+            {
+                FilterAndTransform? list = eventSource._activitySourceSpecs;
+                while (list != null)
                 {
-                    FilterAndTransform? list = eventSource._activitySourceSpecs;
-                    while (list != null)
+                    if ((list.Events & ActivityEvents.ActivityStop) != 0 &&
+                        (activity.Source.Name == list.SourceName || list.SourceName == "*") &&
+                        (list.ActivityName == null || list.ActivityName == activity.OperationName))
                     {
-                        if ((list.Events & ActivityEvents.ActivityStop) != 0 &&
-                            (activity.Source.Name == list.SourceName || list.SourceName == "*") &&
-                            (list.ActivityName == null || list.ActivityName == activity.OperationName))
-                        {
-                            eventSource.ActivityStop(activity.Source.Name, activity.OperationName, list.Morph(activity));
-                            return;
-                        }
-
-                        list = list.Next;
+                        eventSource.ActivityStop(activity.Source.Name, activity.OperationName, list.Morph(activity));
+                        return;
                     }
-                };
 
-                ActivitySource.AddActivityListener(eventSource._activityListener);
+                    list = list.Next;
+                }
             }
 
             // Move all wildcard nodes at the end of the list.
@@ -1067,6 +1085,7 @@ private void Dispose()
                 }
             }
 
+            [RequiresUnreferencedCode(DiagnosticSource.WriteRequiresUnreferencedCode)]
             public List<KeyValuePair<string, string?>> Morph(object? args)
             {
                 // Transform the args into a bag of key-value strings.
@@ -1105,7 +1124,11 @@ private void Dispose()
                                 Interlocked.CompareExchange(ref _implicitTransformsTable,
                                     new ConcurrentDictionary<Type, TransformSpec?>(1, 8), null);
                             }
-                            implicitTransforms = _implicitTransformsTable.GetOrAdd(argType, type => MakeImplicitTransforms(type));
+                            implicitTransforms = _implicitTransformsTable.GetOrAdd(argType, type => MakeImplicitTransformsWrapper(type));
+
+                            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2026:RequiresUnreferencedCode",
+                                Justification = "The Morph method has RequiresUnreferencedCode, but the trimmer can't see through lamdba calls.")]
+                            static TransformSpec? MakeImplicitTransformsWrapper(Type transformType) => MakeImplicitTransforms(transformType);
                         }
 
                         // implicitTransformas now fetched from cache or constructed, use it to Fetch all the implicit fields.
@@ -1145,6 +1168,7 @@ private void Dispose()
 
             // Given a type generate all the implicit transforms for type (that is for every field
             // generate the spec that fetches it).
+            [RequiresUnreferencedCode(DiagnosticSource.WriteRequiresUnreferencedCode)]
             private static TransformSpec? MakeImplicitTransforms(Type type)
             {
                 TransformSpec? newSerializableArgs = null;
@@ -1239,6 +1263,7 @@ public TransformSpec(string transformSpec, int startIdx, int endIdx, TransformSp
             /// if the spec is OUTSTR=EVENT_VALUE.PROP1.PROP2.PROP3 and the ultimate value of PROP3 is
             /// 10 then the return key value pair is  KeyValuePair("OUTSTR","10")
             /// </summary>
+            [RequiresUnreferencedCode(DiagnosticSource.WriteRequiresUnreferencedCode)]
             public KeyValuePair<string, string?> Morph(object? obj)
             {
                 for (PropertySpec? cur = _fetches; cur != null; cur = cur.Next)
@@ -1289,6 +1314,7 @@ public PropertySpec(string propertyName, PropertySpec? next)
                 /// Given an object fetch the property that this PropertySpec represents.
                 /// obj may be null when IsStatic is true, otherwise it must be non-null.
                 /// </summary>
+                [RequiresUnreferencedCode(DiagnosticSource.WriteRequiresUnreferencedCode)]
                 public object? Fetch(object? obj)
                 {
                     PropertyFetch? fetch = _fetchForExpectedType;
@@ -1331,6 +1357,7 @@ public PropertyFetch(Type? type)
                     /// <summary>
                     /// Create a property fetcher for a propertyName
                     /// </summary>
+                    [RequiresUnreferencedCode(DiagnosticSource.WriteRequiresUnreferencedCode)]
                     public static PropertyFetch FetcherForProperty(Type? type, string propertyName)
                     {
                         if (propertyName == null)
@@ -1379,11 +1406,7 @@ public static PropertyFetch FetcherForProperty(Type? type, string propertyName)
                         }
                         else
                         {
-                            [UnconditionalSuppressMessage("ReflectionAnalysis", "IL2070:RequiresUnreferencedCode",
-                                Justification = "If the property is trimmed, a message gets logged to inform the user.")]
-                            static PropertyInfo? GetProperty(TypeInfo type, string name) => type.GetDeclaredProperty(name);
-
-                            PropertyInfo? propertyInfo = GetProperty(typeInfo, propertyName);
+                            PropertyInfo? propertyInfo = typeInfo.GetDeclaredProperty(propertyName);
                             if (propertyInfo == null)
                             {
                                 Logger.Message($"Property {propertyName} not found on {type}. Ensure the name is spelled correctly. If you published the application with PublishTrimmed=true, ensure the property was not trimmed away.");