Extension method for a function

Per this question, I think the answer is "no".

I would advise you go with a static include of Retry, as you suggested:

Retry(() => GetUserId("Email"), 2);

It makes intent clear, it's uncomplicated, it's readable enough, and it's idiomatic C#.

An idea that I don't like:

If you were willing to reverse your method arguments, the following would work (but I think most people would think it's pretty awful):

public static T AttemptsAt<T>(this int maxAttempts, Func<T> func)
{
    for (int i = 0; i < maxAttempts; i++)
    {
        try
        {
            return func();
        }
        catch
        {

        }
    }
    throw new Exception("Retries failed.");
}

Usage:

var userId = 2.AttemptsAt(() => GetUserId("Email"));

The problem is that the member method cannot implicitly cast itself to a Func - which to me seems odd, but maybe there is a good explanation :).

Anyway here is how i would call a Func extension:

var userId = ((Func<int>)GetUserId("Email")).Retry(2);

Of course if you need a one-liner, you will have to cast explicitly to the desired delegate type:

var userId = ((Func<int>)(() => GetUserId("Email"))).Retry(2);