Conflict between System.Runtime versions 4.1.2.0 and 4.2.1.0

I'm trying to build a net472 project (Winforms) which references a number of other class library projects in the same solution -- a mixture of net472 and netstandard2.0. I'm getting a compiler warning:

> Found conflicts between different versions of "System.Runtime" that could not be resolved. These reference conflicts are listed in the build log when log verbosity is set to detailed.

With detailed log verbosity, I have this:

> There was a conflict between "System.Runtime, Version=4.1.2.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" and "System.Runtime, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".

The warning says it's on file **"C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets"** and it's on line 2110. That file doesn't contain any instances of "System.Runtime", "4.1.2.0", or "4.2.1.0". Here is what it looks like at line 2110:


I can't make sense of what it's telling me is wrong on line 2110. Furthermore, none of my project files nor the App.config contain any reference to System.Runtime. I'm at a loss as to how to clear this warning. Any help would be greatly appreciated.

I'm on VS 2017 Pro 15.9.11.