dotnet/reactive · error · ArgumentNullException

observer

Error message

observer

What it means

AsyncObserver.IsEmpty(observer) throws ArgumentNullException with paramName "observer" when the downstream IAsyncObserver<bool> is null. It composes Any over a negated Select on the given observer and requires a valid downstream observer.

Solutions

1. Pass the actual downstream observer (from SubscribeSafeAsync) into AsyncObserver.IsEmpty.
2. Fix the custom observable implementation so the observer received in SubscribeAsync is forwarded, not a null field.
3. Add your own ArgumentNullException guard when defining operator overloads.

Example fix

// before
return source.SubscribeSafeAsync(AsyncObserver.IsEmpty(_observer)); // _observer is null
// after
return source.SubscribeSafeAsync(AsyncObserver.IsEmpty(observer));

if (observer is null) throw new ArgumentNullException(nameof(observer));
// before calling AsyncObserver.IsEmpty<TSource>(observer)

static bool HasObserver(IAsyncObserver<bool> o) => o is not null;

try { var o = AsyncObserver.IsEmpty<int>(observer); }
catch (ArgumentNullException ex) when (ex.ParamName == "observer") { /* fix observer forwarding */ }

When it happens

Trigger: Calling AsyncObserver.IsEmpty<TSource>(null) while authoring custom observers or wiring a custom IAsyncObservable where the downstream observer is null.

Common situations: Custom operator implementations that forget to forward the observer parameter, or custom observers holding a nullable observer field that was never assigned.

Related errors

• Value cannot be null. (Parameter 'observer')
• observer
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/34df4703de3e97f6. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/IsEmpty.cs:23

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<bool> IsEmpty<TSource>(this IAsyncObservable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return Create<TSource, bool>(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.IsEmpty<TSource>(observer)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> IsEmpty<TSource>(IAsyncObserver<bool> observer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return Any<TSource>(Select<bool, bool>(observer, b => !b));
        }
    }
}

View on GitHub (pinned to 94b5d5ab91)