Non-generic StringComparer conversion for dict and hashset

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/Dictionary.cs

@@ -54,7 +54,7 @@ public Dictionary(int capacity, IEqualityComparer<TKey>? comparer)
                 Initialize(capacity);
             }
 
-            if (comparer != null && comparer != EqualityComparer<TKey>.Default) // first check for null to avoid forcing default comparer instantiation unnecessarily
+            if (comparer is not null && comparer != EqualityComparer<TKey>.Default) // first check for null to avoid forcing default comparer instantiation unnecessarily
             {
                 _comparer = comparer;
             }
@@ -65,18 +65,11 @@ public Dictionary(int capacity, IEqualityComparer<TKey>? comparer)
 
             if (typeof(TKey) == typeof(string))
             {
-                if (_comparer is null)
-                {
-                    _comparer = (IEqualityComparer<TKey>)NonRandomizedStringEqualityComparer.WrappedAroundDefaultComparer;
-                }
-                else if (ReferenceEquals(_comparer, StringComparer.Ordinal))
-                {
-                    _comparer = (IEqualityComparer<TKey>)NonRandomizedStringEqualityComparer.WrappedAroundStringComparerOrdinal;
-                }
-                else if (ReferenceEquals(_comparer, StringComparer.OrdinalIgnoreCase))
-                {
-                    _comparer = (IEqualityComparer<TKey>)NonRandomizedStringEqualityComparer.WrappedAroundStringComparerOrdinalIgnoreCase;
-                }
+                // We know TKey is string from above check so Unsafe.As from
+                // the generic and then from string back to the generic
+                _comparer = Unsafe.As<IEqualityComparer<TKey>>(
+                    NonRandomizedStringEqualityComparer.GetStringComparer(
+                        Unsafe.As<IEqualityComparer<string>>(_comparer)));
             }
         }
 

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/HashSet.cs

@@ -54,7 +54,7 @@ public HashSet() : this((IEqualityComparer<T>?)null) { }
 
         public HashSet(IEqualityComparer<T>? comparer)
         {
-            if (comparer != null && comparer != EqualityComparer<T>.Default) // first check for null to avoid forcing default comparer instantiation unnecessarily
+            if (comparer is not null && comparer != EqualityComparer<T>.Default) // first check for null to avoid forcing default comparer instantiation unnecessarily
             {
                 _comparer = comparer;
             }
@@ -65,18 +65,11 @@ public HashSet(IEqualityComparer<T>? comparer)
 
             if (typeof(T) == typeof(string))
             {
-                if (_comparer is null)
-                {
-                    _comparer = (IEqualityComparer<T>)NonRandomizedStringEqualityComparer.WrappedAroundDefaultComparer;
-                }
-                else if (ReferenceEquals(_comparer, StringComparer.Ordinal))
-                {
-                    _comparer = (IEqualityComparer<T>)NonRandomizedStringEqualityComparer.WrappedAroundStringComparerOrdinal;
-                }
-                else if (ReferenceEquals(_comparer, StringComparer.OrdinalIgnoreCase))
-                {
-                    _comparer = (IEqualityComparer<T>)NonRandomizedStringEqualityComparer.WrappedAroundStringComparerOrdinalIgnoreCase;
-                }
+                // We know T is string from above check so Unsafe.As from
+                // the generic and then from string back to the generic
+                _comparer = Unsafe.As<IEqualityComparer<T>>(
+                    NonRandomizedStringEqualityComparer.GetStringComparer(
+                        Unsafe.As<IEqualityComparer<string>>(_comparer)));
             }
         }
 

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/NonRandomizedStringEqualityComparer.cs

@@ -2,9 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Diagnostics;
-using System.Globalization;
 using System.Runtime.Serialization;
-using Internal.Runtime.CompilerServices;
 
 namespace System.Collections.Generic
 {
@@ -20,9 +18,9 @@ public class NonRandomizedStringEqualityComparer : IEqualityComparer<string?>, I
         // that was passed in to the ctor. The caller chooses one of these singletons so that the
         // GetUnderlyingEqualityComparer method can return the correct value.
 
-        internal static readonly NonRandomizedStringEqualityComparer WrappedAroundDefaultComparer = new OrdinalComparer(EqualityComparer<string?>.Default);
-        internal static readonly NonRandomizedStringEqualityComparer WrappedAroundStringComparerOrdinal = new OrdinalComparer(StringComparer.Ordinal);
-        internal static readonly NonRandomizedStringEqualityComparer WrappedAroundStringComparerOrdinalIgnoreCase = new OrdinalIgnoreCaseComparer(StringComparer.OrdinalIgnoreCase);
+        private static readonly NonRandomizedStringEqualityComparer WrappedAroundDefaultComparer = new OrdinalComparer(EqualityComparer<string?>.Default);
+        private static readonly NonRandomizedStringEqualityComparer WrappedAroundStringComparerOrdinal = new OrdinalComparer(StringComparer.Ordinal);
+        private static readonly NonRandomizedStringEqualityComparer WrappedAroundStringComparerOrdinalIgnoreCase = new OrdinalIgnoreCaseComparer(StringComparer.OrdinalIgnoreCase);
 
         private readonly IEqualityComparer<string?> _underlyingComparer;
 
@@ -109,5 +107,26 @@ internal override RandomizedStringEqualityComparer GetRandomizedEqualityComparer
                 return RandomizedStringEqualityComparer.Create(_underlyingComparer, ignoreCase: true);
             }
         }
+
+        public static IEqualityComparer<string> GetStringComparer(IEqualityComparer<string>? comparer)
+        {
+            // Special-case EqualityComparer<string>.Default, StringComparer.Ordinal, and StringComparer.OrdinalIgnoreCase.
+            // We use a non-randomized comparer for improved perf, falling back to a randomized comparer if the
+            // hash buckets become unbalanced.
+            if (comparer is null)
+            {
+                comparer = WrappedAroundDefaultComparer;
+            }
+            else if (ReferenceEquals(comparer, StringComparer.Ordinal))
+            {
+                comparer = WrappedAroundStringComparerOrdinal;
+            }
+            else if (ReferenceEquals(comparer, StringComparer.OrdinalIgnoreCase))
+            {
+                comparer = WrappedAroundStringComparerOrdinalIgnoreCase;
+            }
+
+            return comparer;
+        }
     }
 }