TF215097 error when using a custom build activity

Whist trying to make use of a custom build activity I got the error

TF215097: An error occurred while initializing a build for build definition \Tfsdemo1\Candy: Cannot create unknown type '{clr-namespace:TfsBuildExtensions.Activities.CodeQuality;assembly=TfsBuildExtensions.Activities.StyleCop}StyleCop'

This occurred when the TFS 2010 build controller tried to parse the build process .XAML at the start of the build process. A check of all the logs gave no other information other than this error message, nothing else appeared to have occurred.

If I removed the custom activity from the build process all was OK and the build worked fine.

So my initial though was that the required assembly was not loaded into source control and the ‘version control path to custom assemblies’ set. However on checking the file was there and the path set.

What I had forgotten was that this custom activity assembly had a reference to a TfsBuildExtensions.Activities assembly that contained a base class. It was not that the named assembly was missing but that it could not be loaded because a required assembly was missing. Unfortunately there was no clue to this in the error message or logs.

So if you see this problem check for references you might have forgotten and make sure ALL the required assemblies are loaded into source control on the control path for custom assemblies used by the build controller