GAC – Assembly is in the GAC but “Could not load file or assembly”

Question

Hi,

I have an Interop dll that was generated by Visual Studio for a third-party COM object that I am consuming in a .NET dll.

I have registered both my consuming dll and the Interop dll in the GAC. I have to use the GAC because these DLLs are being used by a SharePoint 2010 workflow.

When the execution gets to the point where my dll calls the Interop dll the following error was thrown” “Could not load file or assembly” … “The system could not find the file specified” together with the expected version and public key.

If I examine the Fusion Assembly Binding Log Viewer, the following error is listed in the log entry of the Interop dll:

LOG: GAC Lookup was unsuccessful.

I can see that assembly in the GAC, and it has the correct version and public key token as specified in the FileNotFound exception.

What’s going on?

Answer 1

Hope your third-party COM object is also registered on the machine

Answer 2

The problem is that, as well as Version and Public Key being valid, the Processor Architecture of the Interop must match that of your calling DLL. And, of course, the Target Framework must be the same version.

My dll was compiled to the MSIL (Agnostic, AnyCPU) processor, but for some strange reason Visual Studio insisted on compiling the Interop to x386.

(Maybe this wouldn’t normally cause an issue, but my SharePoint server is 64 bit, which may have caused the symptoms to appear).

The solution is to compile the Interop yourself:

1 - Unregister your dll and the Interop from the GAC.

2 - Start the Visual Studio Command Prompt of the Visual Studio version that relates to the .NET framework your dll is targeting (i.e. I was developing my dll in Visual Studio 2010, targeting .NET 3.5. To do this step I needed to start the Visual Studio 2008 Command Prompt)

3 - Generate the Interop using Tlbmp

tlbimp <full path and filename of  COM .tlb>  /out:c:\.\Interop.CoolThirdParty.dll /keyfile: <full path and filename of snk> /machine:Agnostic /Namespace:CoolThirdParty  

The /machine:Agnostic causes the Interop to be built targeting the MSIL Processor Architecture, which is the same as my dll.

4 - Remove the old reference to the Interop in your dll project

5 - Delete the old Interop file and replace it with the one that you have just generated (e.g. c:.\Interop.CoolThirdParty.dll)

6 - Add a reference in your project to the new Interop.

7 - Rebuild

8 - Register your newly build dll and the new Interop in the GAC

It should start working.