NullReferenceException potential

[bradphelan]

I've modified some code in RUI only to get a null reference exception because a Schedular could not be found. Tracing back to where the null came from

static RxApp()
{
    TaskpoolScheduler = Scheduler.TaskPool;

    DeferredScheduler = new WaitForDispatcherScheduler(() => {
        Type dispatcherType = 
            Type.GetType("System.Reactive.Windows.Threading.DispatcherScheduler, System.Reactive.Windows.Threading", false) ?? 
            Type.GetType("System.Reactive.Windows.Threading.CoreDispatcherScheduler, System.Reactive.Windows.Threading", false);

        if (dispatcherType != null) {
            return (IScheduler)dispatcherType.GetProperty("Current").GetMethod.Invoke(null, null);
        }

        return null;
    });

has a fallthrough at the end to return null if not Scheduler could be found. I suspect that the return null at the end was just to make the compiler happy that all code paths were covered but is really an error.

Question: Is it a valid code path for this to return null? If not can we throw an exception here.

The code where the NullReferenceException pops up is here

    public class WaitForDispatcherScheduler : IScheduler
    {
        ...
        public IDisposable Schedule<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
        {
            return attemptToCreateScheduler().Schedule(state, action);
        }

I would argue that

return attemptToCreateScheduler().Schedule(state, action);

is evil. attempt suggests that there may be failure but the return value is not checked and thus a NullReferenceException will occur.

I won't fix it straight away. Will wait for feedback as I'm not really sure what the behaviour should be for this code.

BTW. Life would be easier if we used the F# option class to indicate an empty return rather than null. I have my own implementation of it in C# called Maybe. Makes the code cleaner and you can clearly specify via types that you might return something or nothing

Maybe<IScheduler> attemptCreateScheduler(){}

and you can consume it a number of ways

return attemptCreateScheduler().Else<SomeException>(); 
return attemptCreateScheduler().Else(Scheduler.Default);
return attemptCreateScheduler().Else(()=>Scheduler.Default);

The point is you have to go through hoops to get a null reference exception when using Maybe.

[bradphelan]

Something evil is going on in the tests. The above code generates me a NRE when I run the test BindToShouldntInitiallySetToNull by itself but the code passes when I run all the tests together. Note I stashed all my changes and this failure is on rxui5-master.

Test Name:  BindToShouldntInitiallySetToNull
Test FullName:  ReactiveUI.Tests.PropertyBindingTest.BindToShouldntInitiallySetToNull
Test Source:    c:\Users\phelan\workspace\ReactiveUI\ReactiveUI.Tests\PropertyBindingTest.cs : line 342
Test Outcome:   Failed
Test Duration:  0:00:38,01

Result Message: System.NullReferenceException : Object reference not set to an instance of an object.
Result StackTrace:  
at ReactiveUI.WaitForDispatcherScheduler.Schedule[TState](TState state, Func`3 action) in c:\Users\phelan\workspace\ReactiveUI\ReactiveUI\WaitForDispatcherScheduler.cs:line 27
   at System.Reactive.Concurrency.Scheduler.Schedule(IScheduler scheduler, Action action)
   at System.Reactive.Linq.Observαble.Return`1._.Run()
   at System.Reactive.Linq.Observαble.Return`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.TailRecursiveSink`1.MoveNext()
   at System.Reactive.Concurrency.AsyncLock.Wait(Action action)
   at System.Reactive.TailRecursiveSink`1.<Run>b__0(Action self)
   at System.Reactive.Concurrency.Scheduler.<Schedule>b__45(Action`1 _action, Action`1 self)
   at System.Reactive.Concurrency.Scheduler.<>c__DisplayClass50`1.<InvokeRec1>b__4d(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.TailRecursiveSink`1.Run(IEnumerable`1 sources)
   at System.Reactive.Linq.Observαble.Concat`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Select`2.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.TailRecursiveSink`1.MoveNext()
   at System.Reactive.Concurrency.AsyncLock.Wait(Action action)
   at System.Reactive.TailRecursiveSink`1.<Run>b__0(Action self)
   at System.Reactive.Concurrency.Scheduler.<Schedule>b__45(Action`1 _action, Action`1 self)
   at System.Reactive.Concurrency.Scheduler.<>c__DisplayClass50`1.<InvokeRec1>b__4d(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.TailRecursiveSink`1.Run(IEnumerable`1 sources)
   at System.Reactive.Linq.Observαble.Concat`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Switch`1._.OnNext(IObservable`1 value)
   at System.Reactive.Linq.Observαble.Select`2._.OnNext(TSource value)
   at System.Reactive.Linq.Observαble.Return`1._.Invoke()
   at System.Reactive.Concurrency.Scheduler.Invoke(IScheduler scheduler, Action action)
   at System.Reactive.Concurrency.ImmediateScheduler.Schedule[TState](TState state, Func`3 action)
   at System.Reactive.Concurrency.Scheduler.Schedule(IScheduler scheduler, Action action)
   at System.Reactive.Linq.Observαble.Return`1._.Run()
   at System.Reactive.Linq.Observαble.Return`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Select`2.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Switch`1._.Run()
   at System.Reactive.Linq.Observαble.Switch`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Where`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Select`2.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.DistinctUntilChanged`2.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Select`2.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.CombineLatest`3._.Run()
   at System.Reactive.Linq.Observαble.CombineLatest`3.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.ObservableExtensions.SubscribeSafe[T](IObservable`1 source, IObserver`1 observer)
   at System.Reactive.Linq.Observαble.Where`1.Run(IObserver`1 observer, IDisposable cancel, Action`1 setSink)
   at System.Reactive.Producer`1.Run(IScheduler _, State x)
   at System.Reactive.Concurrency.ScheduledItem`2.InvokeCore()
   at System.Reactive.Concurrency.ScheduledItem`1.Invoke()
   at System.Reactive.Concurrency.CurrentThreadScheduler.Trampoline.Run(SchedulerQueue`1 queue)
   at System.Reactive.Concurrency.CurrentThreadScheduler.Schedule[TState](TState state, TimeSpan dueTime, Func`3 action)
   at System.Reactive.Concurrency.LocalScheduler.Schedule[TState](TState state, Func`3 action)
   at System.Reactive.Producer`1.SubscribeRaw(IObserver`1 observer, Boolean enableSafeguard)
   at System.Reactive.Producer`1.Subscribe(IObserver`1 observer)
   at System.ObservableExtensions.Subscribe[T](IObservable`1 source, Action`1 onNext, Action`1 onError)
   at ReactiveUI.PropertyBinderImplementation.bindToDirect[TTarget,TValue](IObservable`1 This, TTarget target, Expression`1 property, Func`1 fallbackValue) in c:\Users\phelan\workspace\ReactiveUI\ReactiveUI\PropertyBinding.cs:line 1260
   at ReactiveUI.PropertyBinderImplementation.OneWayBind[TViewModel,TView,TVMProp,TVProp](TViewModel viewModel, TView view, Expression`1 vmProperty, Expression`1 viewProperty, Func`1 fallbackValue, Object conversionHint) in c:\Users\phelan\workspace\ReactiveUI\ReactiveUI\PropertyBinding.cs:line 1057
   at ReactiveUI.BindingMixins.OneWayBind[TViewModel,TView,TVMProp,TVProp](TView view, TViewModel viewModel, Expression`1 vmProperty, Expression`1 viewProperty, Func`1 fallbackValue, Object conversionHint) in c:\Users\phelan\workspace\ReactiveUI\ReactiveUI\PropertyBinding.cs:line 238
   at ReactiveUI.Tests.PropertyBindingTest.BindToShouldntInitiallySetToNull() in c:\Users\phelan\workspace\ReactiveUI\ReactiveUI.Tests\PropertyBindingTest.cs:line 346

[bradphelan]

I've tried this a number of times. run all the test passes. run selected test the test fails. The unit test runner should not have any state between tests.

[anaisbetts]

I suspect that it's because the ReactiveUI Service Locator setup gets to run, which reconfigures DeferredScheduler.

[bradphelan]

I'm not really sure what that means. How to fix it?

[carlowahlstedt]

I seem to be running across this issue in my code for Xamarin.iOS. Was there ever a determined resolution?

[anaisbetts]

@carlowahlstedt This is a separate issue with some versions of Xamarin.iOS where Type.GetType doesn't work like it does on other platforms. Paste this into your AppDelegate:

public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
    // NB: GrossHackAlertTiem™:
    //
    // Monotouch appears to not load assemblies when you request them
    // via Type.GetType, unlike every other platform (even
    // Xamarin.Android). So, we've got to manually do what RxUI and
    // Akavache would normally do for us
    var r = new ModernDependencyResolver();
    (new ReactiveUI.Registrations()).Register((f,t) => r.Register(f, t));
    (new ReactiveUI.Cocoa.Registrations()).Register((f,t) => r.Register(f, t));
    (new ReactiveUI.Mobile.Registrations()).Register((f,t) => r.Register(f, t));

    RxApp.DependencyResolver = r;
    (new Akavache.Registrations()).Register(r.Register);
    (new Akavache.Mobile.Registrations()).Register(r.Register);
    (new Akavache.Sqlite3.Registrations()).Register(r.Register);
}

[jlaanstra]

This should be fixed by using the above code. Closing this.