I'm looking at asp.net vnext engineering guideline and have noticed that they recommend to use
Updated: Jan 12, 2016
NotNullAttribute is gone. The ASP.NET team removed it and currently there's no plan to bring it back.
The attribute will be replaced through a precompilation step, using Roslyn, by code that does the actual check.
However, the feature is not yet ready. It will come in a later version.