If a Unity Project needs access to a part of the .NET class library API that is not compiled by default, the Project can inform the C# compiler in Unity. The behavior depends which .NET profile the Project uses.
If your Project uses the .NET Standard 2.0 API Compatibility Level, you don’t need to take any additional steps to use part of the .NET class library API. If part of the API seems to be missing, it might not be included with .NET Standard 2.0. The Project might need to use the .NET 4.x API Compatibility Level instead.
By default, Unity references the following assemblies when you use the .NET 4.x API Compatibility Level:
Use a csc.rsp file to reference any other class library assemblies. You can add this file to the Assets directory of a Unity Project, and use it to pass additional command line arguments to the C# compiler. For example, if your Project uses the HttpClient class, which is defined in the System.Net.Http.dll assembly, the C# compiler might produce this initial error message:
The type `HttpClient` is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
To resolve this error, add the following csc.rsp file to the Project:
-r:System.Net.Http.dll
You should reference class library assemblies as described in the example above. You must place the assembly files in your Project’s Assets folder, or a subfolder of it.
You should be careful when you use a csc.rsp file to reference class library assemblies. If you change the API Compatibility Level from .NET 4.x to .NET Standard 2.0, and a csc.rsp like the one in the example above exists in the Project, then C# compilation fails. The System.Net.Http.dll assembly does not exist in the .NET Standard 2.0 profile, so the C# compiler is unable to locate it.
The csc.rsp file can have parts that are specific to the current .NET profile. If you make changes to the profile, you need to modify the csc.rsp file.