My application has been working with ESRI Silverlight API 3.1 & Silverlight 4 for over a year--and I have been using VS 2012 as my IDE for over 2 years with no problems. Recently, I decided to upgrade my project to Silverlight 5. When I do this, if I continue to use ESRI's Silverlight API 3.1, my project compiles successfully (using Visual Studio 2012) , but an error appears in the "Errors" window with a description of "[Line 0 Position 0]" and all of my ESRI controls (Map, Navogation, etc.) have a blue, squiggly underline in the XAML editor. Is there a known cause for this? I've not been able to find any mention of it here or on Stack Overflow.