Observable.FromEvent throws in v1.0.10425

Question

I'm having trouble using the new Observable.FromEvent with typical WPF events - here's my code:

 

Observable.FromEvent<SizeChangedEventHandler, SizeChangedEventArgs>(
 x => SizeChanged += x, x => SizeChanged -= x)
 .Select(_ => Unit.Value);

 

And here's the result:

 

System.ArgumentException: Error binding to target method.

 at System.Delegate.CreateDelegate(Type type, Object firstArgument, MethodInfo method, Boolean throwOnBindFailure)
 at System.Delegate.CreateDelegate(Type type, Object firstArgument, MethodInfo method)
 at System.Reactive.Linq.Observable.<>c__DisplayClass2c0`2.<FromEvent>b__2be(IObserver`1 observer)
 at System.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
 at System.Reactive.Linq.Observable.<>c__DisplayClass413`2.<Select>b__411(IObserver`1 observer)
 at System.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
 at System.Reactive.Linq.Observable.<>c__DisplayClass302`1.<>c__DisplayClass304.<Merge>b__2fc(IObservable`1 innerSource)
 at System.Reactive.AnonymousObserver`1.Next(T value)
 at System.Reactive.AbstractObserver`1.OnNext(T value)
 at System.Reactive.AnonymousObservable`1.AutoDetachObserver.Next(T value)
 at System.Reactive.AbstractObserver`1.OnNext(T value)
 at System.Reactive.Linq.Observable.<>c__DisplayClass29e`1.<Subscribe>b__29d(Action self)
 at System.Reactive.Concurrency.Scheduler.<Schedule>b__0(Action`1 _action, Action`1 self)
 at System.Reactive.Concurrency.Scheduler.<>c__DisplayClassb`1.<InvokeRec1>b__8(TState state1)
 at System.Reactive.Concurrency.Scheduler.InvokeRec1[TState](IScheduler scheduler, Pair`2 pair)
 at System.Reactive.Concurrency.ImmediateScheduler.Schedule[TState](TState state, Func`3 action)
 at System.Reactive.Concurrency.Scheduler.Schedule[TState](IScheduler scheduler, TState state, Action`2 action)
 at System.Reactive.Concurrency.Scheduler.Schedule(IScheduler scheduler, Action`1 action)
 at System.Reactive.Linq.Observable.Subscribe[TSource](IEnumerable`1 source, IObserver`1 observer, IScheduler scheduler)
 at System.Reactive.Linq.Observable.<>c__DisplayClass2d7`1.<ToObservable>b__2d6(IObserver`1 observer)
 at System.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
 at System.Reactive.Linq.Observable.<>c__DisplayClass302`1.<Merge>b__2fb(IObserver`1 observer)
 at System.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
 at System.Reactive.Linq.Observable.<>c__DisplayClass413`2.<Select>b__411(IObserver`1 observer)
 at System.Reactive.AnonymousObservable`1.<>c__DisplayClass1.<Subscribe>b__0()
 at System.Reactive.Concurrency.Scheduler.Invoke(IScheduler scheduler, Action action)
 at System.Reactive.Concurrency.ScheduledItem`2.InvokeCore()
 at System.Reactive.Concurrency.ScheduledItem`1.Invoke()
 at System.Reactive.Concurrency.CurrentThreadScheduler.Trampoline.Run()
 at System.Reactive.Concurrency.CurrentThreadScheduler.Schedule[TState](TState state, TimeSpan dueTime, Func`3 action)
 at System.Reactive.Concurrency.CurrentThreadScheduler.Schedule[TState](TState state, Func`3 action)
 at System.Reactive.Concurrency.Scheduler.Schedule(IScheduler scheduler, Action action)
 at System.Reactive.AnonymousObservable`1.Subscribe(IObserver`1 observer)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext, Action`1 onError, Action onCompleted)
 at System.ObservableExtensions.Subscribe[TSource](IObservable`1 source, Action`1 onNext)
 at ReactiveUI.Sample.Views.BlockTimerWindow..ctor(BlockItem Model) in C:\Users\Administrator\Dropbox\ReactiveUI_External\ReactiveUI.Sample\ReactiveUI.Sample\Views\BlockTimerWindow.xaml.cs:line 44

 

Any clues?

 

Answer 1

I duplicated the issue, but then changed it to use Observable.FromEventPattern instead, and it works.

Answer 2

The code for FromEvent should be something like this: 

 

public static IObservable<TEventArgs> FromEvent<TDelegate, TEventArgs>(Action<TDelegate> addHandler, Action<TDelegate> removeHandler) where TEventArgs : EventArgs
{
 if (addHandler == null)
 {
  throw new ArgumentNullException("addHandler");
 }
 if (removeHandler == null)
 {
  throw new ArgumentNullException("removeHandler");
 }
 return Observable.Create<TEventArgs>(delegate(IObserver<TEventArgs> observer)
 {
  EventHandler<TEventArgs> firstArgument = new EventHandler<TEventArgs>((_, args) => observer.OnNext(args));
  TDelegate d = (TDelegate)(object)Delegate.CreateDelegate(typeof(TDelegate), firstArgument, typeof(EventHandler<TEventArgs>).GetMethod("Invoke"));
  addHandler(d);
  return Disposable.Create(delegate
  {
   removeHandler(d);
  });
 });
}

 

Answer 3

Another thread http://social.msdn.microsoft.com/Forums/en-US/rx/thread/4f6c28ce-4ae9-470b-99bc-182e1afa712e mentions that FromEvent doesn't work with WPF in the latest drop. User FromEventPattern instead.

 

 

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Dan Sullivan

Answer 4

Use FromEvent for events structurally don't look like a .NET event pattern (i.e. not based on sender, event args), and use FromEventPattern for the pattern-based ones. The sample code show for FromEvent above violates this distinction and uses an EventHandler inside (which indicates the use of the pattern). Does this make sense?

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using (Microsoft.Sql.Cloud.DataProgrammability.Rx) { Signature.Emit("Bart De Smet"); }