Error Cannot convert lambda expression in subscribe for an IObservable

Question

i am trying to implement a standard drag and drop image in wpf using Rx.

var mouseDown = from evt in Observable.FromEventPattern<MouseButtonEventArgs>(image, "MouseLeftButtonDown")
                          select evt.EventArgs.GetPosition(image);

            var mouseUp = Observable.FromEventPattern<MouseButtonEventArgs>(this, "MouseLeftButtonUp");

            var mouseMove = from evt in Observable.FromEventPattern<MouseEventArgs>(this, "MouseMove")
                            select evt.EventArgs.GetPosition(this);

            var q = from startLocation in mouseDown
                    from endLocation in mouseMove.TakeUntil(mouseUp)
                    select new Point 
                    {
                        X = endLocation.X - startLocation.X,
                        Y = endLocation.Y - startLocation.Y
                    };

            q.ObserveOn(SynchronizationContext.Current).Subscribe(point =>
            {
                Canvas.SetLeft(image, point.X);
                Canvas.SetTop(image, point.Y);
            });

i get the error Error Cannot convert lambda expression to type 'System.IObserver<System.Windows.Point>' because it is not a delegate type

what am i missing ?

Your code works ok for me. Have you referenced Rx-Main and Rx-WPF? — Phil, Feb 26 '12 at 16:16
Please check if your target platform says .NET 4 Client Profile and if so then change it to .NET 4. — Christoph, Feb 27 '12 at 06:44
do you have `System` imported? The `Subscribe` you are using is an extension method defined by a class in that namespace. — Gideon Engelberth, Feb 27 '12 at 17:29
can you please post this as answer, with some more details , so that i can accept it ... — ashutosh raina, Feb 27 '12 at 17:48

Asti · Accepted Answer · 2018-07-27T20:14:23.663

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;

score 24 · Answer 2 · answered Apr 07 '17 at 07:56

24

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.

answered Apr 07 '17 at 07:56

The Senator

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Luqqu · Answer 3 · 2019-01-30T15:19:20.457

2

I just stumbled across this question and wanted to add, that I had to add more References to the Project.
The Reference to System.Reactive.Linq alone was not sufficient in my case.

After adding those three the compiler issue was solved:

System.Reactive.Core
System.Reactive.Interfaces
System.Reactive.Linq

Thanks to @Gideon Engelberths comment and The Senator's answer.

edited Jan 30 '19 at 15:19

answered Jan 30 '19 at 14:45

Luqqu

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Error Cannot convert lambda expression in subscribe for an IObservable

3 Answers3