Q: C#: Class to use with Object How can I create a class (in a DLL) that is used with a specific object? Like for instance, a class that Bitmaps call on to do certain things. e.g.   
Bitmap bmp = new Bitmap;
bmp.ThingToDo():

A: Have a look at Extension Methods.
You can add your method to the class Bitmap like the following:
public static class CustomExtension
{
     public static void ThingToDo(this Bitmap bitmap)
     {
         // add your functionality here and use bitmap
     }
}

Good luck!

A: Here's an example of an Extension Method including the Class and Namespace
using System.Runtime.CompilerServices;

namespace Extensions
{
    public static class BitmapExtensions
    {
        [Extension()]
        public static void MakeMyImageCool(Bitmap img)
        {
            //'# do something with your image here.
        }
    }
}

And then to call it
Bitmap img = MySourceImage;
img.MakeMyImageCool();

Notice how even though there is a parameter request in the method, you are not passing a parameter in the call.  That's because the extension method already uses the "img" as the parameter.
