Support assigning null to non-reference types when HandleNull converter is registered (#46)

Author: rmja

SystemTextJsonPatch.Tests/JsonPatchDocumentJsonObjectTest.cs

@@ -1,6 +1,8 @@
-using System.Linq;
+using System;
+using System.Linq;
 using System.Text.Json;
 using System.Text.Json.Nodes;
+using System.Text.Json.Serialization;
 using SystemTextJsonPatch.Exceptions;
 using SystemTextJsonPatch.Operations;
 using Xunit;
@@ -328,6 +330,46 @@ public void ApplyToModelReplaceNull()
 		Assert.Null(model.CustomData["Email"]);
 	}
 
+	[Fact]
+	public void ApplyToModelReplaceNonReferenceWithNull()
+	{
+		// Arrange
+		var model = new SimpleObject { IntegerValue = 123 };
+		var patch = new JsonPatchDocument<SimpleObject>()
+		{
+			Options = new() { Converters = { new IntJsonConverter() } },
+		};
+
+		patch.Operations.Add(new Operation<SimpleObject>("replace", "/IntegerValue", null, null));
+
+		// Act
+		patch.ApplyTo(model);
+
+		// Assert
+		Assert.Equal(999, model.IntegerValue);
+	}
+
+	class IntJsonConverter : JsonConverter<int>
+	{
+		public override bool HandleNull { get; } = true;
+
+		public override int Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options
+		)
+		{
+			if (reader.TokenType == JsonTokenType.Null)
+			{
+				return 999;
+			}
+
+			return reader.GetInt32();
+		}
+
+		public override void Write(Utf8JsonWriter writer, int value, JsonSerializerOptions options)
+		{
+			writer.WriteNumberValue(value);
+		}
+	}
+
 	[Fact]
 	public void ApplyToModelReplaceWithIgnoringCasing()
 	{

SystemTextJsonPatch/Internal/ConversionResultProvider.cs

@@ -16,8 +16,11 @@ internal static bool TryConvertTo(object? value, Type typeToConvertTo, JsonSeria
 
 		if (value == null)
 		{
-			convertedValue = null;
-			return IsNullableType(typeToConvertTo);
+			if (IsNullableType(typeToConvertTo))
+			{
+				convertedValue = null;
+				return true;
+			}
 		}
 
 		if (typeToConvertTo.IsInstanceOfType(value))