Could not load type from assembly error

Question

If you have one project referencing another project (such as a 'Windows Application' type referencing a 'Class Library') and both have the same Assembly name, you'll get this error. You can either strongly name the referenced project or (even better) rename the assembly of the referencing project (under the 'Application' tab of project properties in VS).

Is the assembly in the Global Assembly Cache (GAC) or any place the might be overriding the assembly that you think is being loaded? This is usually the result of an incorrect assembly being loaded, for me it means I usually have something in the GAC overriding the version I have in bin/Debug.

The solution to this for me was not mentioned above, so I thought I would add my answer to the long tail...

I ended up having an old reference to a class (an HttpHandler) in web.config that was no longer being used (and was no longer a valid reference). For some reason it was ignored while running in Studio (or maybe I have that class still accessible within my dev setup?) and so I only got this error once I tried deploying to IIS. I searched on the assembly name in web.config, removed the unused handler reference, then this error went away and everything works great. Hope this helps someone else.

I had the same issue and for me it had nothing to do with namespace or project naming.

But as several users hinted at it had to do with an old assembly still being referenced.

I recommend to delete all "bin"/binary folders of all projects and to re-build the whole solution. This washed out any potentially outdated assemblies and after that MEF exported all my plugins without issue.

I was getting this error and nothing I found on StackOverflow or elsewhere solved it, but bmoeskau's answer to this question pointed me in the right direction for the fix, which hasn't been mentioned yet as an answer. My answer isn't strictly related to the original question, but I'm posting it here under the assumption that someone having this problem will find there way here through searching Google or something similar (like myself one month from now when this bites me again, arg!).

My assembly is in the GAC, so there is theoretically only one version of the assembly available. Except IIS is helpfully caching the old version and giving me this error. I had just changed, rebuilt and reinstalled the assembly in the GAC. One possible solution is to use Task Manager to kill w3wp.exe. This forces IIS to reread the assembly from the GAC: problem solved.