dotnet/reactive · error · ArgumentNullException

actionAsync

Error message

actionAsync

What it means

StartAsync(Func<Task>, IAsyncScheduler) throws ArgumentNullException when the actionAsync delegate is null. The operator invokes the delegate immediately to obtain the Task representing the work, so a null delegate is rejected during argument validation.

Solutions

1. Pass a non-null Func<Task>
2. Guard the call site if the handler is optional
3. Initialize the delegate before invoking StartAsync

Example fix

// before
StartAsync(handler); // handler is null
// after
if (handler != null) StartAsync(handler);

if (actionAsync == null) throw new ArgumentNullException(nameof(actionAsync));

static bool IsValidActionAsync(Func<Task> f) => f is not null;

When it happens

Trigger: Calling the Unit-returning StartAsync overload for Task-returning functions with a null Func<Task>.

Common situations: Unassigned async handler fields, factory/lookup returning null, refactoring that removed the actual handler.

Related errors

• null (Parameter 'scheduler')
• null (Parameter 'values')
• action
• functionAsync
• Value cannot be null. (Parameter 'onErrorAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b3b3b3197cc5287d. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/StartAsync.cs:72

            catch (Exception ex)
            {
                return Throw<TSource>(ex);
            }

            return Create<TSource>(async observer =>
            {
                var subscription = await task.ToAsyncObservable(scheduler).SubscribeAsync(observer).ConfigureAwait(false);

                return StableCompositeAsyncDisposable.Create(cancel, subscription);
            });
        }

        public static IAsyncObservable<Unit> StartAsync(Func<Task> actionAsync) => StartAsync(actionAsync, ImmediateAsyncScheduler.Instance);

        public static IAsyncObservable<Unit> StartAsync(Func<Task> actionAsync, IAsyncScheduler scheduler)
        {
            if (actionAsync == null)
                throw new ArgumentNullException(nameof(actionAsync));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            Task task;

            try
            {
                task = actionAsync();
            }
            catch (Exception ex)
            {
                return Throw<Unit>(ex);
            }

            return task.ToAsyncObservable(scheduler);
        }

        public static IAsyncObservable<Unit> StartAsync(Func<CancellationToken, Task> actionAsync) => StartAsync(actionAsync, ImmediateAsyncScheduler.Instance);

View on GitHub (pinned to 94b5d5ab91)