The default Scripting Runtime Version is .NET 4.6. (.NET 3.5 is marked as deprecated.) This option is a per-Project setting that you specify in the Unity Editor Player settings (Edit > Project Settings, then select the Player category), in the Configuration section:
The equivalent scripting API is the PlayerSettings.scriptingRuntimeVersion property.
Important: Changing this property requires an Editor restart because it affects the Editor as well as Players.
By default nothing should change about how Unity behaves or what .NET functionality is available. Once you use .NET 4.6 for a project, you will be able to use C# 6, .NET 4.6 class libraries, and new runtime features in user scripts and precompiled assemblies.
Note The .Net 3.5 scripting runtime is deprecated, but it will be available for Unity 2018.3 and 2018.4 LTS. Please migrate or start new Projects with the .NET 4.x runtime.
All of them, but in different ways:
Editor and Standalone use the new version of Mono when this option is enabled.
All console platforms will only be able to use IL2CPP when targeting the new .NET version.
iOS and WebGL will continue to be IL2CPP only.
Android will continue to support both Mono and IL2CPP.
Other platforms are still undergoing work to support either new Mono or IL2CPP
IL2CPP fully supports the new .NET 4.6 APIs and features.
The internal, automated tests for Unity pass with the new Mono/IL2CPP. Of course, we still expect you may encounter issues. Please file bugs for any issues you encounter.
No. This is an upgrade of the class libraries and runtime, but we are still using the Boehm GC. We are targeting a new version of Boehm that we use with IL2CPP (so IL2CPP and Mono will have exact same GC).
The newer Mono garbage collector (SGen) requires additional work in Unity and will follow once we have stabilized the new runtime and class libraries.
VSTU 3.1 is required for our new Mono. Please install it to use the new Mono runtime on Windows.
The .NET 4.6 class libraries are quite larger than our current .NET 3.5 class libraries. We are actively working on improving the managed linker to reduce size further.
Additionally, we are working on a new Unity specific class library API profile (like our current unity profile) that will:
a) be specifically implemented to work on AOT platforms
b) smaller in surface area and internally designed to be strippable/linkable
c) support netstandard 2.0 (yet to be officially released)
Please file a bug. We will be rapidly fixing them!