JsonSerializer.Serialize throws when encountering nullable type in netstandard code paths

[chucker]

Minimal repro:

1. Create a .NET Framework app
2. Reference S.T.Json 3.2
3. Run this code:

    class Program
    {
        static void Main(string[] args)
        {
            var json = JsonSerializer.Serialize(new Foo
            {
                Anchor = new MyStruct { X = 1 }
            });

            Console.WriteLine(json);
        }
    }

    struct MyStruct
    {
        public int X { get; set; }
    }

    class Foo
    {
        public MyStruct? Anchor { get; set; }
    }

Exception:

Unhandled Exception: System.ArgumentException: GenericArguments[0], 'System.Nullable`1[ConsoleApp12.MyStruct]', on 'System.Func`2[System.Object,TProperty] CreateStructPropertyGetter[TClass,TProperty](GetPropertyByRef`2)' violates the constraint of type 'TClass'. ---> System.Security.VerificationException: Method System.Text.Json.ReflectionMemberAccessor.CreateStructPropertyGetter: type argument 'System.Nullable`1[ConsoleApp12.MyStruct]' violates the constraint of type parameter 'TClass'.
   at System.RuntimeMethodHandle.GetStubIfNeeded(RuntimeMethodHandleInternal method, RuntimeType declaringType, RuntimeType[] methodInstantiation)
   at System.Reflection.RuntimeMethodInfo.MakeGenericMethod(Type[] methodInstantiation)
   --- End of inner exception stack trace ---
   at System.RuntimeType.ValidateGenericArguments(MemberInfo definition, RuntimeType[] genericArguments, Exception e)
   at System.Reflection.RuntimeMethodInfo.MakeGenericMethod(Type[] methodInstantiation)
   at System.Text.Json.ReflectionMemberAccessor.CreatePropertyGetter[TClass,TProperty](PropertyInfo propertyInfo)
   at System.Text.Json.JsonPropertyInfoCommon`4.Initialize(Type parentClassType, Type declaredPropertyType, Type runtimePropertyType, Type implementedPropertyType, PropertyInfo propertyInfo, Type elementType, JsonConverter converter, JsonSerializerOptions options)

Note that this does not reproduce in netcoreapp3.1 \ net5.0.

---

Describe the bug

When an object that is passed to JSRuntime.InvokeVoidAsync's args contains a property that is a nullable value type (e.g., System.Drawing.Point?), and that property is set to a value, Blazor WASM throws an exception.

To Reproduce

GitHub repo: https://github.com/chucker/blazorjsnullablevaluetype

Add a simple JS logging method to index.html's head:

    <script>
        window.jsTestMethod = (data) => {
            console.log(data);
        };
    </script>

In Index.razor, add a struct, a class that takes the struct as property, and some code that instantiates the class and passes it to the JS method:

@code {
    struct MyStruct
    {
        public int X { get; set; }
    }

    class Foo
    {
        // change this to `MyStruct?` to reproduce
        public MyStruct Anchor { get; set; }
    }

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        var f = new Foo
        {
            Anchor = new MyStruct { X = 1 }
        };

        await JSRuntime.InvokeVoidAsync("jsTestMethod", f);
    }
}

This will work. Your browser will log:

[Log] {anchor: {x: 1}} (localhost, line 14)

Now, change the Anchor property to be nullable:

public MyStruct? Anchor { get; set; }

This will throw an exception (see below) before the JS method ever gets called.

Exception

crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Invalid generic arguments
      Parameter name: typeArguments
System.ArgumentException: Invalid generic arguments
Parameter name: typeArguments
  at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.MakeGenericMethod_impl(System.Reflection.RuntimeMethodInfo,System.Type[])
  at System.Reflection.RuntimeMethodInfo.MakeGenericMethod (System.Type[] methodInstantiation) <0x2c77fd8 + 0x000d0> in <filename unknown>:0 
  at System.Text.Json.ReflectionMemberAccessor.CreatePropertyGetter[TClass,TProperty] (System.Reflection.PropertyInfo propertyInfo) <0x2c77d60 + 0x0003e> in <filename unknown>:0 
  at System.Text.Json.JsonPropertyInfoCommon`4[TClass,TDeclaredProperty,TRuntimeProperty,TConverter].Initialize (System.Type parentClassType, System.Type declaredPropertyType, System.Type runtimePropertyType, System.Type implementedPropertyType, System.Reflection.PropertyInfo propertyInfo, System.Type elementType, System.Text.Json.Serialization.JsonConverter converter, System.Text.Json.JsonSerializerOptions options) <0x2c77778 + 0x00094> in <filename unknown>:0 
  at System.Text.Json.JsonClassInfo.CreateProperty (System.Type declaredPropertyType, System.Type runtimePropertyType, System.Type implementedPropertyType, System.Reflection.PropertyInfo propertyInfo, System.Type parentClassType, System.Text.Json.Serialization.JsonConverter converter, System.Text.Json.JsonSerializerOptions options) <0x2aed3f0 + 0x002c4> in <filename unknown>:0 
  at System.Text.Json.JsonClassInfo.AddProperty (System.Type propertyType, System.Reflection.PropertyInfo propertyInfo, System.Type classType, System.Text.Json.JsonSerializerOptions options) <0x2aecf00 + 0x00072> in <filename unknown>:0 
  at System.Text.Json.JsonClassInfo..ctor (System.Type type, System.Text.Json.JsonSerializerOptions options) <0x2adb2a0 + 0x00168> in <filename unknown>:0 
  at System.Text.Json.JsonSerializerOptions.GetOrAddClass (System.Type classType) <0x2ad8d30 + 0x00036> in <filename unknown>:0 
  at System.Text.Json.JsonPropertyInfo.get_RuntimeClassInfo () <0x2c77260 + 0x0001a> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.HandleObject (System.Text.Json.JsonPropertyInfo jsonPropertyInfo, System.Text.Json.JsonSerializerOptions options, System.Text.Json.Utf8JsonWriter writer, System.Text.Json.WriteStack& state) <0x2c74cc0 + 0x001fc> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.WriteObject (System.Text.Json.JsonSerializerOptions options, System.Text.Json.Utf8JsonWriter writer, System.Text.Json.WriteStack& state) <0x2c748b8 + 0x000cc> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.Write (System.Text.Json.Utf8JsonWriter writer, System.Int32 originalWriterDepth, System.Int32 flushThreshold, System.Text.Json.JsonSerializerOptions options, System.Text.Json.WriteStack& state) <0x2afbce0 + 0x000ec> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.WriteCore (System.Text.Json.Utf8JsonWriter writer, System.Object value, System.Type type, System.Text.Json.JsonSerializerOptions options) <0x2ad8840 + 0x0008a> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.WriteCore (System.Text.Json.PooledByteBufferWriter output, System.Object value, System.Type type, System.Text.Json.JsonSerializerOptions options) <0x28d4418 + 0x00040> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.WriteCoreString (System.Object value, System.Type type, System.Text.Json.JsonSerializerOptions options) <0x28d4020 + 0x0003c> in <filename unknown>:0 
  at System.Text.Json.JsonSerializer.Serialize[TValue] (TValue value, System.Text.Json.JsonSerializerOptions options) <0x28d3bb0 + 0x0000c> in <filename unknown>:0 
  at Microsoft.JSInterop.JSRuntime.InvokeAsync[TValue] (System.String identifier, System.Threading.CancellationToken cancellationToken, System.Object[] args) <0x2c4db10 + 0x001f6> in <filename unknown>:0 
  at Microsoft.JSInterop.JSRuntime.InvokeAsync[TValue] (System.String identifier, System.Object[] args) <0x2c4d420 + 0x0005a> in <filename unknown>:0 
  at Microsoft.JSInterop.JSRuntimeExtensions.InvokeVoidAsync (Microsoft.JSInterop.IJSRuntime jsRuntime, System.String identifier, System.Object[] args) <0x2c4d088 + 0x00042> in <filename unknown>:0 
  at blazorjsnullablevaluetype.Pages.Index.OnAfterRenderAsync (System.Boolean firstRender) <0x2c4c528 + 0x0013e> in <filename unknown>:0 

Further technical details

• ASP.NET Core version Microsoft.AspNetCore.App 3.1.5, Blazor 3.2.0
• Include the output of dotnet --info

> dotnet --info .NET Core SDK (reflecting any global.json): Version: 3.1.301 Commit: 7feb845744

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.15
OS Platform: Darwin
RID: osx.10.15-x64
Base Path: /usr/local/share/dotnet/sdk/3.1.301/

Host (useful for support):
Version: 3.1.5
Commit: 65cd789777

.NET Core SDKs installed:
3.1.301 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.App 3.1.5 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.18 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.19 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

(Windows is also affected.)

• The IDE (VS / VS Code/ VS4Mac) you're running on, and it's version

VS4Mac 8.7 Preview build 1802

(VS 16.7 Preview 3.1 is also affected)

Remarks

• Blazor Server does not appear to be affected
• I have not tried other JS interop methods such as InvokeAsync<>.
• I have not tried whether only properties are affected.

[ShadyNagy]

JsonSerializer.Serialize not working well with nullable or workaround needed here.

that is the problem
https://github.com/dotnet/aspnetcore/blob/c58ab9247c472aa0cb1cb9d71689d044e338c0ee/src/JSInterop/Microsoft.JSInterop/src/JSRuntime.cs#L113

[ShadyNagy]

If struct MyStruct changed to class MyStruct it will work also normally.

[steveharter]

FYI I ran the code on 3.1 (netcoreapp, not netstandard) and although it doesn't throw it does have an issue serializing nullable structs.

The JSON from the test above:

        // 5.0: {"Anchor":{"X":1}}
        // 3.1: {"Anchor":{"HasValue":true,"Value":{"X":1}}}

It appears it is treating Nullable<> as an object and not the special value type it is.

[steveharter]

Per @layomia this is a dup of #30843.

Strategies for workaround:

• Move to 5.0 if possible.
• Use a class, not a struct.
• Write a custom converter for that struct.