System.Text.Json does not seem capable of deserializing nullable custom structs

[andrewjsaid]

Example below, where we see that my object was serialized unconventionally, then could not be deserialized.

First Problem

public class MyClass
{
    public MyDate? Date { get; set; }
}

public struct MyDate
{
    public int Year { get; set; }
}

// Code
var original = new MyClass { Date = new MyDate { Year = 2 } };

var json = System.Text.Json.JsonSerializer.Serialize(original);
Console.WriteLine($"JSON: {json}");

var after = System.Text.Json.JsonSerializer.Deserialize<MyClass>(json);
Console.WriteLine($"Original: {original.Date == null}");
Console.WriteLine($"After: {after.Date == null}");

Output of the program:

JSON: {"Date":{"HasValue":true,"Value":{"Year":2}}}
Original: False
After: True

Found after migrating similar code to ASP.NET Core 3 when my API was not deserializing my nullable structs.

Second Problem
Furthermore, I believe the following is a related bug:

var json2 = "{\"Date\":{\"Year\":2}}";
var after2 = System.Text.Json.JsonSerializer.Deserialize<MyClass>(json2);
Console.WriteLine($"After2: {after.Date == null}"); // Outputs True

From this example we see that System.Text.Json doesn't even expect the "conventional" way of serializing nullables. Does this mean that it just does not support nullable custom structs at all?

Expected
What I expect and what I call the "conventional" way is to output null for any Nullable which is null, and otherwise just write the JSON of X directly (ignoring HasValue and Value). This is the approach of Newtonsoft JSON.NET, as well as what browsers seem to do.

[ahsonkhan]

This is a duplicate of https://github.com/dotnet/corefx/issues/41070.

Does this mean that it just does not support nullable custom structs at all?

Right. We don't have support for nullable custom structs for what shipped in 3.0 and have it on the roadmap for 5.0.

cc @layomia

[RomainHautefeuille]

I can confirm, this is fixed in dotnet 5.0 or System.Text.Json 5.0.0

[andrewjsaid]

Thanks @RomainHautefeuille