When building a mixed solution with C# 4.0 and Silverlight projects, the following error may mislead you to believe something is wrong with the Silverlight project: MSBUILD : error : Copying file C:\Dev\Sources\MyProject\MyProject.Web\debug\bin\MyProject.xap failed. No Silverlight project specified for Silverlight output In fact, what the error means is that the building of a XAP file cannot be done since there are no assemblies in the given output folder. The solution, is changing the Output path...