>>>>>I'm not sure which type it is. Anyway, I dropped them in App_Code, I'm having this problem now
>>>>
>>>>Did you happen to add a reference to the existing project already? If so, remove it. It's either an eternal/referenced DLL or it's part of the same project. You don't want to do both.
>>>
>>>I didn't add a reference except for the fact I added these classes on the Toolbox.
>>
>>Did you drop this control on a form after adding it to the toolbox? If so, the reference was automatically added.
>
>Yes, I did. Here is what I have in the page

What I'm saying here is if you first added their project to the toolbox, then dropped it on a form, the editor will automatically add a reference to this gridview project. If you then added the code from this project to your existing project you now have a conflict between the external reference which was added and the code you just added to your project. You would need to remove the project from References.

>If I would not add this GridView on the toolbox, how would I use this class in my pages?

Take a look at the code that was generated when you dropped it on the form - you can write similar code. However, I'm not saying that you can't have it in the toolbox, only that if you did what I described above, it will cause problems.

>Also why did he name the class GridView and not some custom SortingGridView?
>May be the idea was that having just adding this class to the App_Code all your grids would automatically use this GridView class and not the default GridView?

No idea. I guess so.

>When you tried it yesterday, did you try to create a new project and add it there?

No, I just opened their project to take a quick look at the code.