View component tag helpers don't allow generic parameters

[cjqian]

View components that are invoked with generic parameters will not be translated into view component tag helpers correctly.
#1051

[NTaylorMullen]

This should be done before #5207

[Eilon]

@NTaylorMullen @ajaybhargavb can you describe here what the scenario is? What is a generic parameter in this context?

[cjqian]

Say you have a view component Foo with an invoke method that takes in something like a Dictionary<int, string> bar. In the current implementation, if you try to use a view component in your .cshtml file like <vc:foo bar=..., I'm not confident that it will get processed correctly.

[NTaylorMullen]

Scenario:

Having a ViewComponent with a generic parameter in its Invoke/InvokeAsync results in broken C# being generated by Razor; resulting in a compilation error when trying to serve a view.

Reason

We generate a TagHelperDescriptors based on a ViewComponents Invoke/InvokeAsync method. Razor then consumes these TagHelperDescriptors to render appropriate code for rendering a TagHelper. In order for that piece to work we must generate an actual TagHelper class based off of the TagHelperDescriptor to work at runtime. This means generating public __TypeName__ __PropertyName { get; set;} within an actual TagHelper where the type name and property name are represented as strings in a TagHelperAttributeDescriptor. Today we construct a TagHelperAttributeDescriptor by saying: TagHelperAttributeDescriptor.TypeName = ViewComponent.Parameter[x].GetType().FullName;. The code gen then directly consumes the attribute descriptors type name to generate the get; set; property. Therefore, for generic types the .FullName would result in a back tick being generated.

[Eilon]

Got it, thanks!

[dougbu]

@NTaylorMullen does DefaultViewComponentHelper have any issues w/ generic view component types? I ask because no tests cover the scenario. That lack makes it less clear whether generic view component support was intended (anywhere).

[Eilon]

What exactly is a generic view component? I thought view components by definition need to be concrete, no? How else could you invoke one?

[NTaylorMullen]

@dougbu I gave it a shot and it doesn't look like it can support generic ViewComponents, I've filed an issue. That aside however, this issue is more geared towards non generic ViewComponents where a generic parameter, lets say List<string> causes ViewComponentTagHelpers to fail. Ex:

public class XViewComponent
{
    public string Invoke(List<string> values)
    {
        return string.Join(", ", values);
    }
}

[dougbu]

@Eilon I should have said "ViewComponent with a generic Invoke[Async]() argument". But @NTaylorMullen thanks for checking on a "ViewComponent that is an open generic type".

[dougbu]

Back to this issue, @NTaylorMullen are you looking to support Invoke<T>(List<T> value) or other open generic methods as well? Or is this issue about only parameters of closed generic types in regular 'ole methods?

[NTaylorMullen]

@dougbu Oh I wouldn't expect that to work because that'd expect the ViewComponent invocation system to map parameters provided and then back-map said parameter to the InvokeAsync call. When you try it in practice you get the following:

[rynowak]

Suggestion: When we port TagHelper discovery to use metadata/ISymbol instead of reflection, we just fix this there. ISymbol (Roslyn) already does the correct formatting of type names for code generation