Recently, I attempted to install a NuGet package to a project in Visual Studio and received the following error:

Failed to add reference to 'System.Transactions'. Please make sure that it is in the Global Assembly Cache.

This would happen intermittently with various libraries (especially System.Core and System.Transactions).

Having recently read this post by Scott Hanselman, I checked that the libraries were indeed in the GAC using this command

gacutil /l | find /i "system.core"

which I ran from an elevated Visual Studio Command Prompt and, indeed, System.Core was there.

Some extensive Googling revealed nothing until I tried to add a reference to another project in the same solution, and received this error message:

which lead me to this NuGet issue related to a particular version of Resharper.

I upgraded Resharper to 8.2.1 (from 8.2.0) and everything worked perfectly!

Just in case it saves anyone some time.