FooBar.cs

using System;

class Foo {
}

class Bar : Foo {
}

class Program {
    static void Greet(Foo f) {
	    Console.WriteLine("Hello, Foo");
	}
	
	static void Greet(Bar b) {
	    Console.WriteLine("Hello, Bar");
	}
	
	static void Main(string[] argv) {
	    var f = new Foo();
	    var b = new Bar();
	    Greet(f);
	    Greet(b);
	}
}