Please use the most recent build which has been posted here: https://forum.unity3d.com/threads/upgraded-mono-net-in-editor-on-5-5-0b9.438359/ NOTE: This is an EDITOR ONLY build. The runtime players are not shipped with this release. Please post any issues you have as a new thread on this forum and tag the posts with Mono Upgrade Please backup your project before trying this build. Please see the original preview build post for FAQs: https://forum.unity3d.com/threads/upgraded-mono-net-in-editor-on-5-5-0b4.433541/ Bugs Fixed in this Release: Fix GC error "Too many root sets" (835976) Fix crash during asset liveness processing (838279, 839717) Compile Editor assemblies against .Net 4.6 (837358) Fix references to default .Net assemblies (837358) Fix UnityScript compilation (836115) Important Notes No playback runtimes are included with this release. This means you can not build any player executables. Managed debugging via Visual Studio Tools for Unity or MonoDevelop is *not supported* in this release. The compiler targets the C# 6 language. The compiler *by default* will still target the .Net 2.0/3.5 framework profile we previously supported. However, you can change this to target .Net 4.6. (See below for instructions) The updated Mono garbage collector (SGen GC) *is* enabled in this build, however it is running in a special debug mode drastically impacting it's performance. This release is just for testing for stability not performance.