I had a .net v4.0 project which I later added DevExpress controls to. Using the rather heavy handed approach of copy all the DevExpress dll’s to the bin folder, I started to get the error
Could not load file or assembly ‘Microsoft.Windows.Design.Extensibility, Version=184.108.40.206, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ or one of its dependencies. The system cannot find the file specified
This DevExpress post had a comment that mentioned removing the DevExpress.Xpf.dll files and low and behold error has gone.
Next step is to read up on how best to use these dll’s, most likely in the GAC.