The System.Text.Json serializer for Guid backed objects throws System.ArgumentNullException instead of System.Text.Json.JsonException when the property value is null

[matthewvarley]

Describe the bug

System.Text.Json expects custom converters to throw JsonException or NotSupportedException. With a Guid backed ValueObject, the generated converter will throw ArgumentNullException for when presented with a null value in the raw JSON. This happens because the generated code calls into Guid.Parse instead of Guid.TryParse and then handling the result.

// This is the generated code
public override ObjectType Read(ref global::System.Text.Json.Utf8JsonReader reader, global::System.Type typeToConvert, global::System.Text.Json.JsonSerializerOptions options)
{
     return ObjectType.Deserialize(System.Guid.Parse(reader.GetString()));
}

This is a problem because throwing an ArgumentNullException here will cause ASP.NET WebApi endpoints to generate a 500 response instead of a 400. A minimal repro to generate the ArgumentNullException is included in the steps to reproduce.

Steps to reproduce

using System.Text.Json;
using Vogen;

namespace MinimalRepro
{
    public class Program
    {
        public static void Main()
        {
            // Throws ArgumentNullException instead of JsonException
            JsonSerializer.Deserialize<ObjectContainer>("{\"Ot\":null}");
        }
    }

    public class ObjectContainer
    {
        public required ObjectType Ot { get; set; }
    }

    [ValueObject<Guid>(Conversions.Default)]
    public readonly partial struct ObjectType
    {
    }
}

Expected behaviour

The expectation is for a System.Text.Json.Serialization.JsonConverter to throw System.Text.Json.JsonException. Manually changing the generated read method to something like this generates the expected behavior and triggers ASP.NET to return a 400.

public override ObjectType Read(ref global::System.Text.Json.Utf8JsonReader reader, global::System.Type typeToConvert, global::System.Text.Json.JsonSerializerOptions options)
{
  if (System.Guid.TryParse(reader.GetString(), out Guid g))
  {
    return ObjectType.Deserialize(g);
  }
  else
  {
    throw new System.Text.Json.JsonException("Unable to parse");
  }
}

[SteveDunn]

Thanks very much for the feedback and repro. This has now been fixed - good spot!
It will be in the next release, which is being built and should be available in an hour or so (once all the snapshot tests are finished! 🥱 )