Cache polymorphic properties

src/System.Text.Json/src/System/Text/Json/Serialization/JsonClassInfo.AddProperty.cs

@@ -2,10 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System.Collections.Generic;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Reflection;
 using System.Text.Json.Serialization;
+using System.Threading;
 
 namespace System.Text.Json
 {
@@ -209,18 +210,26 @@ internal JsonPropertyInfo CreateRootObject(JsonSerializerOptions options)
                 options: options);
         }
 
-        internal JsonPropertyInfo CreatePolymorphicProperty(JsonPropertyInfo property, Type runtimePropertyType, JsonSerializerOptions options)
+        internal JsonPropertyInfo GetOrAddPolymorphicProperty(JsonPropertyInfo property, Type runtimePropertyType, JsonSerializerOptions options)
         {
-            JsonPropertyInfo runtimeProperty = CreateProperty(
-                property.DeclaredPropertyType, runtimePropertyType,
-                property.ImplementedPropertyType,
-                property.PropertyInfo,
-                parentClassType: Type,
-                converter: null,
-                options: options);
-            property.CopyRuntimeSettingsTo(runtimeProperty);
+            static JsonPropertyInfo CreateRuntimeProperty(Type runtimePropertyType, (JsonPropertyInfo property, JsonSerializerOptions options, Type classType) arg)
+            {
+                JsonPropertyInfo runtimeProperty = CreateProperty(
+                    arg.property.DeclaredPropertyType, runtimePropertyType,
+                    arg.property.ImplementedPropertyType,
+                    arg.property.PropertyInfo,
+                    parentClassType: arg.classType,
+                    converter: null,
+                    options: arg.options);
+
+                arg.property.CopyRuntimeSettingsTo(runtimeProperty);
+
+                return runtimeProperty;
+            }
+
+            var cache = LazyInitializer.EnsureInitialized(ref property.RuntimePropertyCache, () => new ConcurrentDictionary<Type, JsonPropertyInfo>());
 
-            return runtimeProperty;
+            return cache.GetOrAdd(runtimePropertyType, (type, arg) => CreateRuntimeProperty(type, arg), (property, options, Type));
         }
     }
 }

src/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfo.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Reflection;
 using System.Text.Json.Serialization;
@@ -584,5 +585,7 @@ private void VerifyWrite(int originalDepth, Utf8JsonWriter writer)
                 ThrowHelper.ThrowJsonException_SerializationConverterWrite(ConverterBase);
             }
         }
+
+        public ConcurrentDictionary<Type, JsonPropertyInfo> RuntimePropertyCache;
     }
 }

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleArray.cs

@@ -31,7 +31,7 @@ private static void HandleStartArray(
             }
             else if (state.Current.JsonClassInfo.ClassType == ClassType.Unknown)
             {
-                jsonPropertyInfo = state.Current.JsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, typeof(object), options);
+                jsonPropertyInfo = state.Current.JsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, typeof(object), options);
             }
 
             // Verify that we have a valid enumerable.

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Read.HandleValue.cs

@@ -24,7 +24,7 @@ private static void HandleValue(JsonTokenType tokenType, JsonSerializerOptions o
             }
             else if (state.Current.JsonClassInfo.ClassType == ClassType.Unknown)
             {
-                jsonPropertyInfo = state.Current.JsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, typeof(object), options);
+                jsonPropertyInfo = state.Current.JsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, typeof(object), options);
             }
 
             jsonPropertyInfo.Read(tokenType, ref state, ref reader);

src/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializer.Write.Helpers.cs

@@ -31,7 +31,7 @@ private static void GetRuntimePropertyInfo(object value, JsonClassInfo jsonClass
                 // Nothing to do for typeof(object)
                 if (runtimeType != typeof(object))
                 {
-                    jsonPropertyInfo = jsonClassInfo.CreatePolymorphicProperty(jsonPropertyInfo, runtimeType, options);
+                    jsonPropertyInfo = jsonClassInfo.GetOrAddPolymorphicProperty(jsonPropertyInfo, runtimeType, options);
                 }
             }
         }

src/System.Text.Json/tests/Serialization/Object.WriteTests.cs

@@ -75,5 +75,12 @@ public int this[int index]
 
             public string NonIndexerProp { get; set; }
         }
+
+        [Fact]
+        public static void WriteCollectionAsObject()
+        {
+            string json = JsonSerializer.Serialize(new { Prop = (object)new[] { 0 } });
+            Assert.Equal(@"{""Prop"":[0]}", json);
+        }
     }
 }