Give a base class "Base", I want to write a method Test, like this:

private static bool Test(IEnumerable enumerable)
{
...
}

such that Test returns true if the type of o implements any interface of IEnumerable where X derives from Base, so that if I would do this:

public static IEnumerable<string> Convert(IEnumerable enumerable)
{
    if (Test(enumerable))
    {
        return enumerable.Cast<Base>().Select(b => b.SomePropertyThatIsString);
    }

    return enumerable.Cast<object>().Select(o => o.ToString());
}

...that it would do the right thing, using Reflection. I'm sure that its a matter of walking across all the interfaces of the type to find the first that matches the requirements, but I'm having a hard time finding the generic IEnumerable<> among them.

Of course, I could consider this:

public static IEnumerable<string> Convert(IEnumerable enumerable)
{
    return enumerable.Cast<object>().Select(o => o is Base ? ((Base)o).SomePropertyThatIsString : o.ToString());
}

...but think of it as a thought experiment.