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 Projects use the .NET Standard 2.0 Api Compatibility Level, you shouldn’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 may need to use the .NET 4.x Api Compatibility Level instead.
By default, Unity references the following assemblies when using the .NET 4.x Api Compatibility Level:
You should reference any other class library assemblies using an csc.rsp file. You can add this file to the AssetsAny media or data that can be used in your game or Project. An asset may come from a file created outside of Unity, such as a 3D model, an audio file or an image. You can also create some asset types in Unity, such as an Animator Controller, an Audio Mixer or a Render Texture. More info
See in Glossary directory of a Unity Project, and use it to pass additional command line arguments to the C# compiler. For example, if a 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'.
You can resolve this error by adding 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. Don’t copy them into the Project directory.
Exercise caution when using an 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 an 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.