Question: For the Build Process in TFS 2010 I've created a library containing some custom code activities. 
In the past it all worked fine by adding the library (\*.dll) to Source Control and setting the 'Build Controller - Version Control Path to Custom Assemblies' to the path where the library could be found in Source Control.

But since a few days (and I've been updating the library often) the build doesn't succeed anymore.
The error reported is:

> 
> TF215097: An error occurred while
>  initializing a build for build
>  definition "Cannot create unknown
>  type '{clr-namespace:BuildTasks;assembly=BuildTasks}'"
> 
> 
> 

After [searching](https://stackoverflow.com/questions/1419734/how-to-solve-unknow-type-problem-when-load-a-xmal-file) I couldn't find any other solution than installing the library to the GAC. That works but I wonder why it is impossible to get it to work without having to install in to the GAC. 

So although it working again now, I'd like to get it back to work the old way, without GAC. Hopefully some of you can help me. Thanks in advance.

Answer: 
As mentioned by Rhapsody on June 2 2010, the GAC can be used. However it needs to be noted that if you do use the GAC, you need to stop and restart the build service, so that it re-scans the GAC, hence picking up your DLL.

I had initially registered my DLL in the GAC, and when I kicked off a build that pointed to a workflow consisting of my custom code activity assembly, the build failed. But after stopping and restarting the build service, the build does not fail straight away with the dreaded "An error occurred while initializing a build for build definition [BuildDefinitionName] : Cannot create unknown type...".
I encountered the same issue after a reboot of our TFS 2012 build server. A CI job that was running perfectly fine stopped working with the mentioned TF215097 error, it was unable to find a custom activity.

A comment on this page (<https://social.msdn.microsoft.com/Forums/vstudio/en-US/479449e1-5c02-4744-b620-3bba64038cef/custom-build-activity-tf215097-cannot-create-unknown-type?forum=tfsbuild>) suggested removing the set source path to the custom activity DLLs in the build controller configuration, saving the updated configuration, re-adding the same path and saving again. 

This fixed the issue for me.