dotnet/reactive · error · ArgumentNullException

nameof(subscribeAsync)

Error message

nameof(subscribeAsync)

What it means

Observable.Create<TResult> with an async subscribe delegate (Func<IObserver<TResult>, CancellationToken, Task>) throws ArgumentNullException when subscribeAsync is null. This overload supports async/await-based observables, wiring a CancellationToken that fires on disposal. Rx validates the delegate immediately at Create.

Solutions

1. Pass a valid async lambda: async (o, ct) => { ... }
2. Fix the delegate source that returned null
3. Guard the delegate for null before Create

Example fix

// before
var obs = Observable.Create<int>(subscribeAsync); // subscribeAsync is null
// after
var obs = Observable.Create<int>(async (o, ct) =>
{
    await Task.Yield();
    o.OnNext(1);
    o.OnCompleted();
});

if (subscribeAsync == null) throw new ArgumentNullException(nameof(subscribeAsync));
var obs = Observable.Create<T>(subscribeAsync);

bool IsValid<T>(Func<IObserver<T>, CancellationToken, Task> f) => f is not null;

try { var obs = Observable.Create<T>(subscribeAsync); }
catch (ArgumentNullException ex) when (ex.ParamName == "subscribeAsync") { /* handle null delegate */ }

When it happens

Trigger: Calling Observable.Create<TResult>(subscribeAsync) with a null Func<IObserver<TResult>, CancellationToken, Task>, e.g. an unassigned async method reference or a factory returning null.

Common situations: Async observable construction where the async lambda was refactored into a method that no longer exists, or DI/config-driven delegate selection yielding null.

Related errors

• action (Parameter 'action')
• begin
• end
• nameof(subscribe)
• observableFactory

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/05fb2e24cab7b4cc. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Creation.cs:77

        #endregion

        #region + CreateAsync +

        /// <summary>
        /// Creates an observable sequence from a specified cancellable asynchronous Subscribe method.
        /// The CancellationToken passed to the asynchronous Subscribe method is tied to the returned disposable subscription, allowing best-effort cancellation.
        /// </summary>
        /// <typeparam name="TResult">The type of the elements in the produced sequence.</typeparam>
        /// <param name="subscribeAsync">Asynchronous method used to produce elements.</param>
        /// <returns>The observable sequence surfacing the elements produced by the asynchronous method.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="subscribeAsync"/> is null.</exception>
        /// <remarks>This operator is especially useful in conjunction with the asynchronous programming features introduced in C# 5.0 and Visual Basic 11.</remarks>
        /// <remarks>When a subscription to the resulting sequence is disposed, the CancellationToken that was fed to the asynchronous subscribe function will be signaled.</remarks>
        public static IObservable<TResult> Create<TResult>(Func<IObserver<TResult>, CancellationToken, Task> subscribeAsync)
        {
            if (subscribeAsync == null)
            {
                throw new ArgumentNullException(nameof(subscribeAsync));
            }

            return s_impl.Create(subscribeAsync);
        }

        /// <summary>
        /// Creates an observable sequence from a specified asynchronous Subscribe method.
        /// </summary>
        /// <typeparam name="TResult">The type of the elements in the produced sequence.</typeparam>
        /// <param name="subscribeAsync">Asynchronous method used to produce elements.</param>
        /// <returns>The observable sequence surfacing the elements produced by the asynchronous method.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="subscribeAsync"/> is null.</exception>
        /// <remarks>This operator is especially useful in conjunction with the asynchronous programming features introduced in C# 5.0 and Visual Basic 11.</remarks>
        public static IObservable<TResult> Create<TResult>(Func<IObserver<TResult>, Task> subscribeAsync)
        {
            if (subscribeAsync == null)
            {
                throw new ArgumentNullException(nameof(subscribeAsync));

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