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 in the Silverlight project properties under the Build tab to the main web project instead of “\bin\Debug” and \bin\Release”.
Alternatively you can edit the Silverlight’s .csproj file under both Release and Debug property groups. I.e.: