1. Question:
 platform: Unity3d 2017.3.1 language: C-sharp 
 because you need to reference System.Numerics, but Unity"s solution project does not support direct reference to dll 
, so I found System.Numerics.dll from C:Program Files (x86) Reference AssembliesMicrosoftFramework.NETFrameworkv4.6 
 to import into Assets, and an error occurred. (I have selected Script Runtime Version as .net version 4.6 at Edit-Project Settings-Player-Other Settings) 
2, Code:
 
System.Numerics.BigIntegerdll
3:
 
4. What methods have you tried:
 I have tried to use System.Numerics.dll under .net 3.5 to put it in Assets. 
 has not been resolved yet. 
5. Thank you
C:Program Files (x86) Reference AssembliesMicrosoftFramework.NETFramework
you can find System.Numerics, interested in this directory and import it to Unity, to find the problem.
