Error Cannot convert lambda expression in subscribe for an IObservable

The namespace System.Reactive.Linq contains the static class Observable which defines all the extension methods for common reactive combinators. It resides in System.Reactive.dll

The extension methods for IObservable<T>.Subscribe such as Subscribe(onNext), Subscribe(onNext, onError) are however defined in mscorlib in the static class System.ObservableExtensions.

tl;dr:

• For Rx/Observable extension methods you need to import System.Reactive.Linq = using System.Reactive.Linq;
• For Subscribe overloads you need to import System = using System;