Add typed fast paths in ReadArray for common element types

Avoid Array.CreateInstance + Array.SetValue reflection overhead
for object[], string[], and Type[] arrays in metadata deserialization.

Author: davkean

src/Microsoft.VisualStudio.Composition/Configuration/SerializationContextBase.cs

@@ -57,15 +57,15 @@ internal abstract partial class SerializationContextBase : IDisposable
 
         private static readonly object BoxedTrue = true;
         private static readonly object BoxedFalse = false;
+
+        private static readonly IReadOnlyDictionary<string, object?> EmptyMetadata = ImmutableDictionary<string, object?>.Empty;
         private static readonly object BoxedCreationPolicyAny = CreationPolicy.Any;
         private static readonly object BoxedCreationPolicyShared = CreationPolicy.Shared;
         private static readonly object BoxedCreationPolicyNonShared = CreationPolicy.NonShared;
         private static readonly object BoxedInt32Zero = 0;
         private static readonly object BoxedInt32One = 1;
         private static readonly object BoxedInt32NegativeOne = -1;
 
-        private static readonly IReadOnlyDictionary<string, object?> EmptyMetadata = ImmutableDictionary<string, object?>.Empty;
-
         internal SerializationContextBase(BinaryReader reader, Resolver resolver)
         {
             Requires.NotNull(reader, nameof(reader));
@@ -758,6 +758,41 @@ protected Array ReadArray(BinaryReader reader, Func<object?> itemReader, Type el
                     throw new NotSupportedException();
                 }
 
+                // Use typed fast paths for common element types to avoid
+                // the reflection overhead of Array.CreateInstance + Array.SetValue.
+                if (elementType == typeof(object))
+                {
+                    var array = new object?[(int)count];
+                    for (int i = 0; i < array.Length; i++)
+                    {
+                        array[i] = itemReader();
+                    }
+
+                    return array;
+                }
+
+                if (elementType == typeof(string))
+                {
+                    var array = new string?[(int)count];
+                    for (int i = 0; i < array.Length; i++)
+                    {
+                        array[i] = (string?)itemReader();
+                    }
+
+                    return array;
+                }
+
+                if (elementType == typeof(Type))
+                {
+                    var array = new Type?[(int)count];
+                    for (int i = 0; i < array.Length; i++)
+                    {
+                        array[i] = (Type?)itemReader();
+                    }
+
+                    return array;
+                }
+
                 var list = Array.CreateInstance(elementType, (int)count);
                 for (int i = 0; i < list.Length; i++)
                 {