Question

How to invoke Display method extending object class?

static class Tools
{
    public static void Display<T>(this T t)
    {
        Console.WriteLine("generic: " + t.GetType());

    }

    public static void Display(this object o)
    {
        Console.WriteLine("object: " + o.GetType());
    }
}

class Program
{
    static void Main(string[] args)
    {
        int i = 100;


        // all  will invoke the generic version.
        Tools.Display<int>(i);
        i.Display();
        Tools.Display(i);

    }
}

Answer 1

I can’t remember where in the standard it says it, but C# prefers to call the most specific overload. With generics, the generic version of the function will almost always take preference. So while an int is an object, it better fits the Display<T>(T) than Display(object), since the realization of the generic (Display<int>(int)) is an exact match. Add the fact that C# can figure out which type belongs in the T by itself and you see the behavior you’re experiencing.

So, you must explicitly cast to an object to call the object version:

((object)i).Display();

Alternatively:

Tools.Display((object)i);

---

And you’ll have a curious (but sensible) issue if you do:

object o = 5;
o.Display();
o.Display<object>();

This will call the object version in the first case and the generic one in the second. Fun times with parameters!