C# .NET - Reactive extensions error - Asked By Mitu N on 28-Apr-15 02:38 PM

I've a C# 4.0 class library projet in which I've referenced Reactive extensions dlls(2.2.5 version).
I'm getting compilation errors as mentioned below.Could you advise please?

C# code:
var observable = System.Reactive.Linq.Observable.Empty<bool>();

            foreach (var modelParam in modelParams)
                observable = observable.Merge(modelParam.ObservePropertyChanged(p => p.IsDirty).Where(p => p));

Error 522 'System.IObservable<bool>' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.IObservable<bool>' could be found (are you missing a using directive or an assembly reference?
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Error 508 'System.IObservable<bool>' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.IObservable<bool>' could be found (are you missing a using directive or an assembly reference?)  

using System.Linq;