In the projects I’m doing I often use controls from my own library or use control from third party vendors. I don’t like the process of doing this. Rather I would like to have the libraries available inside Expression Blend, like the controls that came with the Silverlight Toolkit or Blend itself.
One way is to add the custom assemblies containing the controls to one the folders in your Program Files containing the assemblies for the toolkit for example. Another and for more elegant way is to make Expression Blend look in an additional location in your UserProfile (c:/Users/SomeUser/…). The Expression Suite already has a folder in your My Documents, but not for assets.
I’ve create a .BAT file that adds a couple of folders to the Expression folder in the My Documents folder of the current user. Folders to store assemblies for .NET and Silverlight in various versions. A .REG file is used to add these locations to the windows registry.
After you have run both you’ll have to restart Blend to reread the registry and the folders. The assemblies placed in these folders are now available in Expression Blend. Your assemblies should show up in the Assets panel. By dragging and dropping them onto you pages the assemblies are automagically added to your project.
! Disclaimer: Note that by doing this you are making changes to the windows registry. Be sure to backup your system and the registry, and to save your work. I’m not taking any responsibility for you screwing up your system by not knowing what you are doing. Using the .reg and .bat files is at your on risk.