Generics and Inheritance

Consider the following code in C#, it executes just fine. As the middle tier class instantiates the Storage class as an Array of TValue.

       public class Storage<TValue> {
		public TValue Values { get; set; }
	}

	public class BaseClass<TValue> : Storage<TValue[]> { }

	public class FinalClass<TValue> {

		private BaseClass<string> baseClass;

		public FinalClass() => this.baseClass = new BaseClass<string> { Values = new string[] { } }; //Important Line of Code
	}

Because the middle tier class defines the TValue parameter as string, it allows to set the Values property in the FinalClass as new string[]. However, this behavior breaks in Blazor, because of the RuntimeHelpers.TypeCheck method.

Storage.razor

@typeparam TValue

@code {
	
	[Parameter]
	public TValue Values { get; set; }

}

BaseClass.razor

@typeparam TValue
@inherits Storage<TValue[]>

FinalClass.razor

<BaseClass TValue="string" Values="@(new string[]{})"></BaseClass>

The error you receive is “cannot convert from ‘string[]’ to ‘TValue[]’”

[SteveSandersonMS]

The repro for this can be simplified a lot, so it looks worse to me now. All you need is the following component:

    public class SomeComponent<T> : ComponentBase
    {
        [Parameter] public T[] Items { get; set; }
    }

You just can’t use this component. If you don’t specify the T parameter explicitly, the compiler complains that it can’t infer the type and you should specify it. But if you do specify it, like this:

<SomeComponent T="string" Items="@(Array.Empty<string>())"></SomeComponent>

… then you get the same compiler error reported above: Argument 1: cannot convert from 'string[]' to 'T[]'. The generated code looks like this:

__builder.AddAttribute(
    1,
    "Items",
    global::Microsoft.AspNetCore.Components.CompilerServices.RuntimeHelpers.TypeCheck<T[]>(
        Array.Empty<string>()));

… which is wrong because the compiler hasn’t substituted in the known value for T.

I don’t know whether this would be a trivial fix or is just impossible given the design of the type checker, but it’s definitely incorrect, so reopening.

[mkArtakMSFT]

This should be resolved already by #34734