dotnet/reactive · error · ArgumentNullException

ArgumentNullException: source

Error message

ArgumentNullException: source

What it means

ToEventPattern converts an IObservable of EventPattern back into a .NET-style event source, and it throws ArgumentNullException when the source observable is null. Because the entire conversion is driven by the source sequence, a null source cannot produce a meaningful result. The check is eager so the failure surfaces at the call site.

Solutions

1. Ensure the observable passed to ToEventPattern is non-null before calling.
2. If a producer can yield null, substitute Observable.Empty<EventPattern<TSender, TEventArgs>>() or the ?? operator.
3. Move the ToEventPattern call inside the success branch where the source is known to exist.

Example fix

// before
IEventPatternSource<object, RoutedEventArgs> src = maybeObservable.ToEventPattern<object, RoutedEventArgs>(); // maybeObservable may be null
// after
IEventPatternSource<object, RoutedEventArgs> src = (maybeObservable ?? Observable.Empty<EventPattern<object, RoutedEventArgs>>())
    .ToEventPattern<object, RoutedEventArgs>();

if (source is null) source = Observable.Empty<EventPattern<TSender, TEventArgs>>();

static bool IsObservableValid<TSender, TArgs>(IObservable<EventPattern<TSender, TArgs>>? s) => s is not null;

try { var src = observable.ToEventPattern<TSender, TEventArgs>(); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* use default event source */ }

When it happens

Trigger: Calling source.ToEventPattern<TSender, TEventArgs>() where source is a null IObservable<EventPattern<TSender, TEventArgs>>.

Common situations: A factory method or dictionary lookup returned a null observable; a conditional query pipeline short-circuited to null instead of Observable.Empty; refactoring changed a field to nullable without updating call sites.

Related errors

• ArgumentNullException: selector
• ArgumentNullException: source
• Value cannot be null. (Parameter 'asyncOperationSelector')
• Value cannot be null. (Parameter 'resultSelector')
• action

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/fdf7e4732c7fe76f. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive.WindowsRuntime/System.Reactive.Linq/WindowsObservable.Events.cs:114

                {
                    removeHandler(h);
                };
            });
        }

        /// <summary>
        /// Exposes an observable sequence as an object with a typed event.
        /// </summary>
        /// <typeparam name="TSender">The type of the sender that raises the event.</typeparam>
        /// <typeparam name="TEventArgs">The type of the event data generated by the event.</typeparam>
        /// <param name="source">Observable source sequence.</param>
        /// <returns>The event source object.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static IEventPatternSource<TSender, TEventArgs> ToEventPattern<TSender, TEventArgs>(this IObservable<EventPattern<TSender, TEventArgs>> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            return new EventPatternSource<TSender, TEventArgs>(source, static (h, evt) => h(evt.Sender!, evt.EventArgs));
        }
    }
}

View on GitHub (pinned to 94b5d5ab91)