Why is Enumerable.OfType<TResult>(IEnumerable) doesn't work as expected with ValueTuple?

Question

Behavior

So I have a collection of Tuple that looks like this:

public List<(ElementRowViewModel Parent, ElementBase Element)> ResultCollection { get; private set; } = new List<(ElementRowViewModel Parent, ElementBase Element)>();

When I try to do the following

private async Task RegisterAndCommitElements<TElement>() where TElement : ElementBase
{
    var specificElements = this.ResultCollection.OfType<(ElementRowViewModel parent, TElement element)>();
}

It yields nothing. But When I do

 this.ResultCollection[0] is (ElementRowViewModel parent, SpecificElement element)

It returns true, Or even

 this.ResultCollection[0] is (ElementRowViewModel parent, TElement element)

It also returns true.

Am I miss-understanding something? Or is there a limitation or a known bug with Enumerable.OfType(IEnumerable) when using tuple as type parameter?

Answer 1

A ValueTuple<ElementRowViewModel, SpecificElement> value isn't a ValueTuple<ElementRowViewModel, ElementBase>. There's an implicit conversion from one to the other, but that's defined by the C# language (basically it performs element-wise conversion) not by the CLR type system, which is what OfType cares about.

using System;

public class Test
{
    public static void Main(string[] args)
    {
        var strings = ("hello", "there");
        object boxed = strings;
        Console.WriteLine(boxed is ValueTuple<string, string>); // True
        Console.WriteLine(boxed is ValueTuple<object, object>); // False

        // This is still valid though: element-wise implicit conversions.
        (object, object) objects = strings;
    }
}

That second is test that returns false is effectively what the OfType method is doing, so it makes sense that your value isn't yielded.