Cache StringBuilder per thread and remove Lazy from GetRuntimeClassName.

src/WinRT.Runtime/ComWrappersSupport.cs

@@ -12,6 +12,7 @@
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
 using WinRT.Interop;
 
 #if NET
@@ -871,14 +872,23 @@ private static ComInterfaceEntry ProvideIReferenceArray(Type arrayType)
 
         internal sealed class InspectableInfo
         {
-            private readonly Lazy<string> runtimeClassName;
+            private readonly Type _type;
 
             public Guid[] IIDs { get; }
-            public string RuntimeClassName => runtimeClassName.Value;
+
+            private volatile string _runtimeClassName;
+
+            private string MakeRuntimeClassName()
+            {
+                global::System.Threading.Interlocked.CompareExchange(ref _runtimeClassName, TypeNameSupport.GetNameForType(_type, TypeNameGenerationFlags.GenerateBoxedName | TypeNameGenerationFlags.ForGetRuntimeClassName), null);
+                return _runtimeClassName;
+            }
+
+            public string RuntimeClassName => _runtimeClassName ?? MakeRuntimeClassName();
 
             internal InspectableInfo(Type type, Guid[] iids)
             {
-                runtimeClassName = new Lazy<string>(() => TypeNameSupport.GetNameForType(type, TypeNameGenerationFlags.GenerateBoxedName | TypeNameGenerationFlags.ForGetRuntimeClassName));
+                _type = type;
                 IIDs = iids;
             }
         }

src/WinRT.Runtime/TypeNameSupport.cs

@@ -298,19 +298,29 @@ struct VisitedType
         /// Tracker for visited types when determining a WinRT interface to use as the type name.
         /// Only used when GetNameForType is called with <see cref="TypeNameGenerationFlags.ForGetRuntimeClassName"/>.
         /// </summary>
-        private static readonly ThreadLocal<Stack<VisitedType>> VisitedTypes = new ThreadLocal<Stack<VisitedType>>(() => new Stack<VisitedType>());
+        private static readonly ThreadLocal<Stack<VisitedType>> VisitedTypes = new ThreadLocal<Stack<VisitedType>>(() => new Stack<VisitedType>());
+
+        [ThreadStatic]
+        private static StringBuilder? nameForTypeBuilderInstance;
 
         public static string GetNameForType(Type type, TypeNameGenerationFlags flags)
         {
             if (type is null)
             {
                 return string.Empty;
             }
-            StringBuilder nameBuilder = new StringBuilder();
+
+            // Get instance for this thread
+            StringBuilder? nameBuilder = nameForTypeBuilderInstance ?? new StringBuilder();
+            nameBuilder.Clear();
             if (TryAppendTypeName(type, nameBuilder, flags))
             {
-                return nameBuilder.ToString();
-            }
+                var name = nameBuilder.ToString();
+                nameForTypeBuilderInstance = nameBuilder;
+                return name;
+            }
+
+            nameForTypeBuilderInstance = nameBuilder;
             return string.Empty;
         }