Unity Build Error:The type 'Object' is defined in an assembly that is not referenced

[Boo0317]

Unity Version 2018.3.0f2
It always has error when I build unity project like this:
Assets\Microsoft.MixedReality.WebRTC.Unity\Scripts\Media\VideoSource.cs(34,32): error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.
I think I have lost something, but I don't know what is missing.
It worked well on Unity Editor.

[Boo0317]

[eirikhollis]

Is your project setup like this?

Under Edit -> Project Settings -> Other Settings

[djee-ms]

Hi @Boo0317,

The error message points at a missing .NET Standard 2.0 library, which should be installed by Unity if you have the setup that @eirikhollis pointed above.

[Boo0317]

Thank you for your quickly reply.I've tried IL2CPP it build successed, but I want to build an Unity C# project so I selected .NET rather than IL2CPP. Does it only support IL2CPP now?

[djee-ms]

This is a Unity limitation, see e.g. https://forum.unity.com/threads/2018-2-16f1-uwp-net-scripting-backend-net-standard-2-0-build-errors.588733/

We don't support .NET Standard 2.0 on .NET scripting backend. It's only supported when using Mono and IL2CPP.

The .NET backend is deprecated in 2018.3 and completely removed in 2019.1. There is not much I can do about it, you will have to use IL2CPP.