[release/6.0] Fix System.Object serialization with custom number handling

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/ObjectConverter.cs

@@ -7,11 +7,6 @@ namespace System.Text.Json.Serialization.Converters
 {
     internal sealed class ObjectConverter : JsonConverter<object?>
     {
-        public ObjectConverter()
-        {
-            IsInternalConverterForNumberType = true;
-        }
-
         public override object? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
         {
             if (options.UnknownTypeHandling == JsonUnknownTypeHandling.JsonElement)
@@ -50,15 +45,5 @@ internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, object? va
 
             runtimeConverter.WriteAsPropertyNameCoreAsObject(writer, value, options, isWritingExtensionDataProperty);
         }
-
-        internal override object? ReadNumberWithCustomHandling(ref Utf8JsonReader reader, JsonNumberHandling handling, JsonSerializerOptions options)
-        {
-            if (options.UnknownTypeHandling == JsonUnknownTypeHandling.JsonElement)
-            {
-                return JsonElement.ParseValue(ref reader);
-            }
-
-            return JsonNodeConverter.Instance.Read(ref reader, typeof(object), options);
-        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/Object.ReadTests.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Text.Json.Nodes;
 using Xunit;
 
 namespace System.Text.Json.Serialization.Tests
@@ -655,5 +656,20 @@ public static void TooLittleJsonFails(string json)
 
             Assert.Equal(0, reader.BytesConsumed);
         }
+
+        [Theory]
+        [InlineData(JsonUnknownTypeHandling.JsonElement, typeof(JsonElement))]
+        [InlineData(JsonUnknownTypeHandling.JsonNode, typeof(JsonNode))]
+        public static void ReadSystemObjectWithNumberHandling(JsonUnknownTypeHandling unknownTypeHandling, Type expectedType)
+        {
+            var options = new JsonSerializerOptions
+            {
+                NumberHandling = JsonNumberHandling.AllowReadingFromString,
+                UnknownTypeHandling = unknownTypeHandling
+            };
+
+            object result = JsonSerializer.Deserialize<object>(@"{ ""key"" : ""42"" }", options);
+            Assert.IsAssignableFrom(expectedType, result);
+        }
     }
 }

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

@@ -138,5 +138,13 @@ public static void EscapingShouldntStackOverflow()
 
             Assert.Equal("{\"name\":\"\u6D4B\u8A6611\"}", result);
         }
+
+        [Fact]
+        public static void WriteSystemObjectWithNumberHandling()
+        {
+            var options = new JsonSerializerOptions { NumberHandling = JsonNumberHandling.AllowReadingFromString };
+            string result = JsonSerializer.Serialize(new object(), options);
+            Assert.Equal("{}", result);
+        }
     }
 }