C# static classes and the is operator

Question

After refactoring some code recently, which involved some class renames, some of my code broke in a surprising way. The cause was a failing "is" operator test, that I was very surprised wasn't a compiler error or warning.

This complete program shows the situation:

static class ExtensionMethods {}

class Program {

    static void Main() {
        Test("Test");
    }

    public static bool Test(object obj)
    {
        return obj is ExtensionMethods;
    }
}

I would have expected "obj is ExtensionMethods" to raise a warning of some sort, given that ExtensionMethods is a static class.

The compiler will issue a warning for the "is" operator when object under test can never be of the provided type, ((string)obj) is System.Uri for example.

Am I forgetting a scenario in which this would actually be a meaningful test?

Answer 1

I was very surprised wasn't a compiler error or warning.

It should have been. It was an oversight.

There were a number of bugs like that involving static classes. If I recall correctly, there was even some bizarre scenario that Vladimir Reshetnikov found where it was possible to make type inference infer a static type as a bound on a type parameter.

Apparently this one, which I have seen before, never got fixed. Apologies for the oversight.

Am I forgetting a scenario in which this would actually be a meaningful test?

No.