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: 
Check your Windows Event notifications. Your custom activity DLL may NOT have gotten loaded properly. 

If it did not, then you may have to change the platform target of your custom activity project... either to 32bit or 64 bit.
I didn't have any success with the other answers in this thread however I thought I'd share what I did. My custom assembly was loaded onto my build machine through GAC. I had to manually open the build template XAML file and add my assembly to the namespace reference. For some reason Visual Studio wasn't properly referencing this for me. 

Before:

```
xmlns:ba1="clr-namespace:BuildTasks.Activities"

```

After

```
xmlns:ba1="clr-namespace:BuildTasks.Activities;assembly=ModifyTasks"

```

My custom build task assembly is called ModifyTasks.dll replace it with your own file name...