C# Interface Implementation relationship is just “Can-Do” Relationship?

Foo myFoo = new Foo();
return myFoo is IBar;

IBar bar = myArrayList[3] as IBar;
Foo foo = bar as Foo;

void DoSomething(IBar bar) {
}

static void Main() {
    Foo myFoo = new Foo();
    DoSomething(myFoo);
}

interface IBar { void happy(); }
class Foo : IBar
{
    void happy()
    {
        Console.Write("OH MAN I AM SO HAPPY!");
    }
}
class Program
{
    static void Main()
    {
        IBar myBar = new Foo();
        myBar.happy();
    }
}

// Your class implements an interface
class Concrete : ISomeInterface {}

// New up an instance of that class
var myClass = new Concrete();

// You have a method somewhere that takes ISomeInterface as a parameter
public void MyMethod(ISomeInterface blah){}

// You can pass your instance to that method
MyMethod(myClass);