dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'actionAsync')

Error message

Value cannot be null. (Parameter 'actionAsync')

What it means

Observable.StartAsync(Func<Task>) schedules the provided async function on the default concurrency scheduler and converts it into IObservable<Unit>. A null 'actionAsync' delegate throws ArgumentNullException at call time. The check happens eagerly so the failure is attributed to the caller, not to a later subscription.

Solutions

1. Pass a non-null async lambda or method group to StartAsync.
2. Guard the nullable delegate source with an explicit throw or fallback before the call.
3. Register the Func<Task> in DI/config if that is where it comes from.

Example fix

// before
Func<Task> work = _workField; // may be null
var xs = Observable.StartAsync(work);
// after
var work = _workField ?? throw new InvalidOperationException("work not set");
var xs = Observable.StartAsync(work);

if (actionAsync is null)
    throw new ArgumentNullException(nameof(actionAsync));
Observable.StartAsync(actionAsync);

bool IsValidDelegate<T>(T? d) where T : Delegate => d is not null;

try
{
    var xs = Observable.StartAsync(actionAsync);
}
catch (ArgumentNullException ex) when (ex.ParamName == "actionAsync")
{
    // log: async delegate was null
}

When it happens

Trigger: Calling Observable.StartAsync(Func<Task>) with a null delegate: an unassigned Func<Task> field, a factory method returning null, or a conditional lambda assignment with a missing branch.

Common situations: Passing a method group whose instance is null, DI/config-resolved delegates that were never registered, or refactorings that moved the async lambda into a nullable property.

Related errors

• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• nameof(onError)
• nameof(onCompleted)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/8bc69fbb0b6a1f4c. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Async.cs:1281

        }

        /// <summary>
        /// Invokes the asynchronous action, surfacing the result through an observable sequence.
        /// </summary>
        /// <param name="actionAsync">Asynchronous action to run.</param>
        /// <returns>An observable sequence exposing a Unit value upon completion of the action, or an exception.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="actionAsync"/> is null.</exception>
        /// <remarks>
        /// <list type="bullet">
        /// <item><description>The action is started immediately, not during the subscription of the resulting sequence.</description></item>
        /// <item><description>Multiple subscriptions to the resulting sequence can observe the action's outcome.</description></item>
        /// </list>
        /// </remarks>
        public static IObservable<Unit> StartAsync(Func<Task> actionAsync)
        {
            if (actionAsync == null)
            {
                throw new ArgumentNullException(nameof(actionAsync));
            }

            return s_impl.StartAsync(actionAsync);
        }

        /// <summary>
        /// Invokes the asynchronous action, surfacing the result through an observable sequence.
        /// </summary>
        /// <param name="actionAsync">Asynchronous action to run.</param>
        /// <param name="scheduler">Scheduler on which to notify observers.</param>
        /// <returns>An observable sequence exposing a Unit value upon completion of the action, or an exception.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="actionAsync"/> is null or <paramref name="scheduler"/> is null.</exception>
        /// <remarks>
        /// <list type="bullet">
        /// <item><description>The action is started immediately, not during the subscription of the resulting sequence.</description></item>
        /// <item><description>Multiple subscriptions to the resulting sequence can observe the action's outcome.</description></item>
        /// </list>
        /// </remarks>

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