GAC

GAC what do you mean GAC?

The GAC or Global Assebly Cache is the playground of anything .NET it is not language specific so whether you are a VB or C# bod your code used the GAC.

Put simply the GAC is where all the framework and links is stored for anything .NET, this is the reason why you build your website, to assemble all the bits into and from the GAC into the necessary .NET structure.

For example when you add an element to your page using the toolbox Visual Studio puts the necessary connections into the GAC to allow your Visual Studio UI to know about the control.

So if you have anytime not been able to see your control in the code side of Visual Studio, first try a rebuild and if that fails delete it from the source code and readd it using the Toolbox.