// Let's have a class. It's sealed for some reason so we can't extend it.
public sealed class Foo
{
   // This class has two properties, but has nothing useful here.
   public int Value1 {get;set;}
   public int Value2 {get;set;}
}


// let's say I want to add the two numbers together. Because this is a Complicated Operation (tm), I'm gonna write an extension method to do it.


public static class HandyStuff
{
    public static int SumThings(this Foo x)
    {
        return x.Value1 + x.Value2;
    }
}

// let's get an instance...
var foo = new Foo { Value1 = 3, Value2 = 4 };

// Now, I can do stuff like this:
var result = foo.SumThings();