Bizarre Errors When Using Telerik Controls

I’m in the process of developing a complete line-of-business application in Silverlight.  I have a Telerik Premium license and most of the controls I use are Telerik controls.

At one point in my development process, I innocently added an “Up/Down” control to a UserControl, using Blend 4.  Suddenly and for no apparent reason, I could no longer see the UserControl in Visual Studio 2010.  I would get a very odd looking blank page.  I could still edit it in Blend, however, and I’m under a time crunch so I carried on.

Today, Blend started crashing when trying to open the UserControl.  I now had no design surface and I’m struggling with other issues.  Among other strange issues, this error appeared in the VS designer:

An unhandled exception was encountered while trying to render the current silverlight project on the design surface. To diagnose this failure, please try to run the project in a regular browser using the silverlight developer runtime.

If I reloaded the designer I would go back to the blank page.  After much gnashing of teeth, I tried copying this control to another project in my solution.   After commenting out a few lines, Voila!  Design surface!

The Solution: When I used Blend to add the “up/down” control, it added a reference to a prior version of the Telerik.Windows.Controls.Input assembly to my project.  I had several other Telerik assemblies referenced, all Q3 2010 version.  The mismatched versions played hell with the designer and I was stuck.  I removed the offending prior versions and replaced them with the correct ones, and life was good.  Hope this saves you some time and gray hairs.