Error Cannot convert lambda expression in subscribe for an IObservable<Point>

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;

To make this a clearer answer based on @Gideon Engelberths comment 5th down in the question I was missing the 'using System;' using directive in my class:

using System.Reactive.Linq;
using System;

Which then fixed up the compiler issue. Thanks Gideon.