[FIXED] Unity cannot find System.Runtime for stub dlls

I'm attempting to build and use a UWP plugin with Unity. I'm creating a stub DLL for Mono. Unity gives me this error as soon as it is brought into the project.

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When I build, two more errors appear.

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What am I doing wrong with this stub plugin? I'm trying to use 4 versions of this plugin: Mono stub, x86, ARM, and x64. On Unity 5.2.3p3.

 

This is the entire code contained in the dll.

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Code (csharp):

1. using System;
2.  
3. namespace WinFacePlugin
4. {
5.     public static class Face
6.     {
7.         public static void Initialize( Action<Action> winThreadRunner, Action<Action> unityThreadRunner ) {
8.             // Editor stub.
9.         }
10.  
11.         public static void Login( Action<bool, string> callback ) {
12.             // Editor stub.
13.         }
14.     }
15. }

 

The error was because I made my stub .dll using a Windows Universal project. targeting .NET 4.6.1.

I changed to making a "Classic Desktop" library project, and set my target framework to "Unity 3.5 .net Subset Base Class Libraries". There were a few Unity options, not sure which one is the best to pick.