When we open Silverlight 4 application in Silverlight 5 we get following errors
1> error : Unable to read the project file ‘Silverlight4App.csproj’.
2> Silverlight4App.csproj(137,3): The imported project “C:\Program Files\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.CSharp.targets” was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.
This error occurs as the application was earlier built using Silverlight-4 Reference. So while opening this in Silverlight-5, the Visual studio looks for the Silverlight 5 reference files but it cannot find it.
To resolve this error you will have to follow following steps:
1> Open the project in Visual studio.
2> Go to solution explorer
3> Right click on the project that is not loaded and click on edit the CSPROJ file
4> And then change the Target Frame Work version to 5.0
5> Save and close this file and again right click on the project which is not loaded and click on Reload project.
6> Now the project will be reloaded successfully.
By following above steps your Silverlight 4 application will be converted to Silverlight 5 application.
If in case you again want to go back to Silverlight 4 then just repeat the above steps and set the Target Framework version to “v4.0”.
Hope this helps!!