From microsoft and i agree, you need the correct version from the gac and reference it into your project. Once you add the reference you need to set "copy to local" as false ;)
this will work and what i would do if i ran into any dll issue!
step by step:
To copy the dll, goto start - select or search for "run" and copy past this code:
add reference to your project, right click on the reference Microsoft.SharePoint.Sandbox.dll and click on "properties" set copy to local = false
build and run project, should work fine :)