<%@ Register TagPrefix="CustomValidators" Namespace="CustomValidators" Assembly="filename_of_DLL_file" %>

Which I change to:

<%@ Register TagPrefix="cv" Namespace="MyCustomValidators" Assembly="~/bin/CustomValidationControls.dll" %>

However as soon as I compiled the page I received: The given assembly name or codebase was invalid. (Exception from HRESULT: 0×80131047) After searching Google some more I wasn’t any better off with a solution. What is wrong with my code base? Recompiling didn’t help renaming it didn’t help. However after some careful reading of the on-line help related to the @ Register directive I found this statement:

assembly

The assembly in which the namespace that you are associating with the tagprefix attribute resides.

Note
The assembly name does not include a file extension. …

That was the clue I need combined with another post I had read. I had added the full path and extension for the assembly but what it was looking for was ONLY the assembly name it self with no extension or directory path. .net assumes that the path for the assembly will be the bin directory. So the correct @ Register directive should be:

<%@ Register TagPrefix="cv" Namespace="MyCustomValidators" Assembly="CustomValidationControls" %>

As soon as I did that the page compiled with out errors and I now can validate my CheckBoxList.

As an aside note you can also solve this by putting the source file in the App_Code directory and then register the tag prefix this way:

<%@ Register TagPrefix="cv" Namespace="MyCustomValidators" Assembly="__Code" %>