dotnet/reactive · error · ArgumentNullException

action

Error message

action

What it means

Start(Action) throws ArgumentNullException when the action delegate passed to it is null. The library validates arguments eagerly so the failure surfaces at subscription-setup time rather than later inside the reactive pipeline. Since Start wraps the delegate in ToAsync, a null delegate cannot be meaningfully executed.

Solutions

1. Ensure a non-null Action delegate is passed to Start(action)
2. If the delegate may be absent, guard the call site before invoking Start
3. Check why the delegate source (field, factory, dictionary lookup) produced null

Example fix

// before
Action work = GetHandler(); // may be null
var xs = Start(work);
// after
Action work = GetHandler() ?? (() => { });
var xs = Start(work);

if (action == null) throw new ArgumentNullException(nameof(action)); // or guard before calling Start

static bool IsValidAction(Action a) => a is not null;

When it happens

Trigger: Calling System.Reactive.Async Linq Operators.Start with a null Action, e.g. a delegate variable that was never assigned or a method group that resolved to null.

Common situations: Conditionally assigned delegates, refactored code where a callback field is no longer initialized, passing a result of a factory/lookup that returned null.

Related errors

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

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/dfe153663ef556ca. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Start.cs:32

                throw new ArgumentNullException(nameof(function));

            return ToAsync(function)();
        }

        public static IAsyncObservable<TSource> Start<TSource>(Func<TSource> function, IAsyncScheduler scheduler)
        {
            if (function == null)
                throw new ArgumentNullException(nameof(function));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return ToAsync(function, scheduler)();
        }

        public static IAsyncObservable<Unit> Start(Action action)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            return ToAsync(action)();
        }

        public static IAsyncObservable<Unit> Start(Action action, IAsyncScheduler scheduler)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return ToAsync(action, scheduler)();
        }
    }
}

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