Symptom:

We ran into a strange issue recently. We had deployed a signed DLL to the Global Assembly Cache (GAC), and assumed that it would be available to all applications on  the web server.

For some reason, it turned out that one of these DLLs in the GAC was not being resolved by our SharePoint application unless the dll was copied into the \bin folder of the application as well.

The above shows the error that the Sharepoint Application was throwing if the DLL was in the GAC only, but not copied over to the \bin folder of  the virtual.

Resolution:

After some research, we determined that we had to add a line to the web.config of our SharePoint web application, as follows:

The copy of the DLL could then be removed from the \bin folder leaving only the one copy in the GAC.

Hope this helps other developers.

- Rajiv