Argument (nuget): Argument validation methods, for example: this.user = Argument.NotNull("user", user).  Less fancy than approaches based on lambdas/anonymous classes, but less scary performance-wise (given that argument checks are often all over the place).  Annotations: C# 8 nullable reference types, ReSharper, Microsoft.CodeQuality.Analyzers, Code Contracts (.NET 4.5 only).
Keywords: contracts|validation|argument