if your having trouble like this with your Visual Studio 2010 while you have to target .net 3.5, you can do following steps
1. Unload your project within visual studio 2010
2. Edit your project
3. add below code within your project
<Reference Include="System" />
<Reference Include="System.Core">
<RequiredTargetFramework>3.5</RequiredTargetFramework>
</Reference>
