Wrong inferred type param when passing this to Microsoft.JSInterop.DotNetObjectRef

[chucker]

Preview 6 replaces DotNetObjectRef with a generic DotNetObjectRef<T>. Instead of a constructor, the Create<T> method is now used.

When passing this from within a component, this leads to unexpected results due to Razor's parsing behavior.

To Reproduce

Steps to reproduce the behavior:

1. Using ASP.NET Core 3.0 Preview 6 and Visual Studio 2019 16.2 Preview 4
2. Create a new Blazor component MySampleComponent.razor
3. Place this code inside its @code section:

    private DotNetObjectRef<MySampleComponent> _Instance;

    protected override async Task OnAfterRenderAsync()
    {
        _Instance = DotNetObjectRef.Create(this);
    }

4. See error

Expected behavior

This should compile. The type parameter of Create<T> should be inferred as MySampleComponent.

Actual behavior

This doesn't work, because Razor internally renames your class to __generated__MySampleComponent. Consequently, the Error List will show:

CS0029 Cannot implicitly convert type Microsoft.JSInterop.DotNetObjectRef<__generated__MySampleComponent> to Microsoft.JSInterop.DotNetObjectRef<MySampleComponent>

If you explicitly specify the type parameter, the error changes to CS1503.

Workaround

You can declare the variable with the generated class name:

    private DotNetObjectRef<__generated__MySampleComponent> _Instance;

    protected override async Task OnAfterRenderAsync()
    {
        _Instance = DotNetObjectRef.Create(this);
    }

However, this is presumably not the intended behavior.

[chucker]

I filed this a bit prematurely. The above error messages are real. However, when actually building the project, you need to go with the original code.

So, while this will show an error during Razor compilation:

    private DotNetObjectRef<MySampleComponent> _Instance;

    protected override async Task OnAfterRenderAsync()
    {
        _Instance = DotNetObjectRef.Create(this);
    }

, it is in fact the correct syntax for building the project.

[mkArtakMSFT]

Thanks for contacting us, @chucker.
This will be resolved as part of #4065