I was setting up a new build server for a .NET project as I have done several times before. It was a TeamCity build server but that is not important for this problem. When trying to build my solution I got this error:
error CS0012: The type 'IEnumerable<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=126.96.36.199, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
Locally the solution compiled just fine. After some research I found some hints that installing the Targeting Pack for my .NET Framework version solves the problem.
I downloaded and installed the target pack for .NET 4.6 from here. Immediately the problem was solved and the solution compiled without any problems.
It compiled locally because the missing DLL that is installed by the Targeting Pack is also installed by Visual Studio. This is just another reason to think about the “no Visual Studio on the build server” rule.