.NET 5.0 System.Text.Json Serialize/Deserialize throw exception

[jacksonwj]

Here is class and extend function

// class
public class Res
{
    public int Code { get; set; }
    public string Msg { get; set; }

    public Res(int code = 200, string msg = "Success")
    {
        Code = code;
        Msg = msg;
    }
}

public class Res<T> : Res
{
    public T Body { get; set; }

    public Res(T body = default, int code = 200, string msg = "Success")
        : base(code, msg)
    {
        Body = body;
    }
}

// Extend function ToJson()
public static class Ext
{
    private static readonly JsonSerializerOptions _opts;

    static Ext()
    {
        _opts = new JsonSerializerOptions
        {
            // ... default configuration
        };
    }

    // Serialize to JSON string from entity
    public static string ToJson(this object data, JsonSerializerOptions options = null)
    {
        if (data == null)
        {
            return string.Empty;
        }

        return JsonSerializer.Serialize(data, options ?? _opts);   // throw exception when data has Value Type body
    }

    // Deserialize to entity from JSON string
    public Static T ToJson<T>(this string json, JsonSerializerOptions options = null)
    {
        if (string.IsNullOrWhiteSpace(json))
        {
            return default;
        }

        return JsonSerializer.Deserialize<T>(json, options ?? _opts);   // throw exception when data has Value Type body
    }
}

When I serialize an entity to JSON string by System.Text.Json.JsonSerializer (.NET 5.0), it throw an exception.
Deserialize also has the same exception.

var res0 = new Res();
var res0Json = res0.ToJson();   // Success (Correct JSON string)

var res1 = new Res<string>("a");   // body is Reference Type
var res1Json = res1.ToJson();   // Success (Correct JSON string)

var res2 = new Res<int>(1);   // body is Value Type
var res2Json = res2.ToJson();   // Fail (Throw an exception: Object reference not set to an instance of an object.)

It seems that: Value Type CANNOT Serialize/Deserialize, but Reference Type CAN

How do I resolve the exception?

Thanks

[huoyaoyuan]

It should be an internal bug. NullReferenceException should never be thrown in library code. Arguments verification failure should throw other exceptions.

[jacksonwj]

It should be an internal bug. NullReferenceException should never be thrown in library code. Arguments verification failure should throw other exceptions.

The code CAN RUN without exception in .NET Core 3.1 or before.

[benaadams]

Duplicate of #43757

Workarounds #43757 (comment)

Fix #43831

[layomia]

Closing as dup of #43757. I'll propose porting the fix (#43831) to a servicing release for .NET 5.0.