JsonSerializer.Deserialize on Nullable value types

[amosonn]

Related issues: #434, #1256

System.Text.Json JsonSerializer.DeserializeAsync fails to deserialize Nullable value types. Running:

static void Main(string[] args)
{
    var stream = new MemoryStream(Encoding.UTF8.GetBytes(@"{ ""SubObj"": {""Field1"": ""value1""}, ""Field2"": ""value2"" }"));
    var des = JsonSerializer.DeserializeAsync<Obj2>(stream).Result;
    System.Console.WriteLine(des.Field2);
    System.Console.WriteLine(des.SubObj.Value.Field1);
}

private struct Obj1
{
    public string? Field1 { get; set; }
}

private class Obj2
{
    public string? Field2 { get; set; }
    public Obj1? SubObj { get; set; }
}

Prints:

value2
Unhandled exception. System.InvalidOperationException: Nullable object must have a value.
   at System.Nullable`1.get_Value()
   at test_deser.Program.Main(String[] args) in /tmp/test_deser/Program.cs:line 15

Removing the ? and the Value works as expected:

-    System.Console.WriteLine(des.SubObj.Value.Field1);
+    System.Console.WriteLine(des.SubObj.Field1);

-    public Obj1? SubObj { get; set; }
+    public Obj1 SubObj { get; set; }

Prints:

value2
value1

On netcore 3.1.3. Using nullable types syntax, but this displays the same behavior with explicit Nullable<Obj1>.

[layomia]

This is fixed in 5.0, following #2259. However, porting the fix to 3.x will not meet the servicing bar.

Some workarounds here are to use a custom converter, upgrade the app to a preview of .NET 5, or use a preview of the System.Text.Json v5.0.0 package on NuGet.