NuGet: 4.1.0
>PM Install-Package System.Reactive -Version 4.1.0
Version 4.1.0 is an enhancement version over 4.0.0 by keeping the public API and extending it additively as well as having significant internal changes for better performance via less allocation, less indirection and less locking on the existing components. Note that the codebase now uses C# 7.1 language features.