The type or namespace name XXX could not be found(are you missing using directive or an assembly reference?)

No.
Normally, just the reference and the using are enough - there is nothing else you should need to add.

Place the cursor in the name of the class you are trying to use. Does a small red line appear at the right hand side of the name?
If is does, hover the mouse over it and open the drop down when it appears. That will offer you options for solving the problem.

If it doesn't, does it appear at the left hand side?
If it does, hover the mouse again, and open the drop down. If this is basically "create the class" than the reference is not in properly - double check your references and the DLL / namespace for the class.

If all this fails, try a full re-build and see what happens.

I can only conclude that you haven't referenced all the correct DLLs as you don't need to anything else.

There are a few gotchas to be aware of

The same namespace can be used in multiple dll's.

If you have written a project (call it projectA) that calls system.web and then write another project (call it projectB) that references projectA, you will also need to add a reference to system.web in projectB too.

I copied an example of implementing image slide show from the internet and had the same problem. I got the refered error although in my case the code was not in App_Code folder. The code that was causing this error was in a folder that contained the user control. Because of the way compiler works the page with control was being compiled before the code of the control and so the referenced code could not be found resulting in error!

I therefore went to the folder with the control, right clicked the the control and chose the build page option thereby compiling the control code. Note the compiler does not recompile a code unless there has been changes in that code. Running the web site did not give the error.

Samuel Kamau

Open your project file with a text editor and verify that the RootNamespace element is correct as well as the AssemblyName element. I ran into this problem after renaming an existing project. Apparently Visual Studio doesn't handle such things very well.