dotnet/reactive · error · ArgumentNullException

ArgumentNullException: action

Error message

ArgumentNullException: action

What it means

WindowsRuntimeThreadPoolScheduler.Schedule<TState> requires a non-null scheduling delegate; it throws ArgumentNullException named 'action' when the Func<IScheduler, TState, IDisposable> passed to schedule work on the WinRT thread pool is null. The library validates up front because a null delegate cannot be wrapped in the UserWorkItem that runs on ThreadPool.RunAsync.

Solutions

1. Pass a valid Func<IScheduler, TState, IDisposable> delegate or lambda to Schedule.
2. Check any variable holding the delegate for null before calling Schedule.
3. If the action may be absent, return a no-op scheduler lambda (e.g. (_, _) => Disposable.Empty) instead of null.

Example fix

// before
scheduler.Schedule(state, GetAction()); // GetAction() may return null
// after
var action = GetAction() ?? ((sch, st) => Disposable.Empty);
scheduler.Schedule(state, action);

if (action is null) throw new ArgumentNullException(nameof(action)); // or fix at call site:
// scheduler.Schedule(state, action ?? ((s, st) => Disposable.Empty));

bool IsValidAction<TState>(Func<IScheduler, TState, IDisposable>? a) => a is not null;

try { scheduler.Schedule(state, action); }
catch (ArgumentNullException ex) when (ex.ParamName == "action") { /* supply valid delegate or skip */ }

When it happens

Trigger: Calling scheduler.Schedule(state, null) or passing a null delegate variable/ternary result as the action argument.

Common situations: Delegates built conditionally (e.g. cond ? action1 : null), methods that forward an optional action parameter, or refactored code where the lambda was accidentally removed.

Related errors

• action (Value cannot be null)
• ArgumentNullException (default message: Value cannot be…
• scheduler
• scheduler
• scheduler (Value cannot be null)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/b5c7f71d84a8e215. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive.WindowsRuntime/System.Reactive.Concurrency/WindowsRuntimeThreadPoolScheduler.cs:82

        public WorkItemPriority Priority { get; }

        /// <summary>
        /// Gets the options that configure how work is scheduled.
        /// </summary>
        public WorkItemOptions Options { get; }

        /// <summary>
        /// Schedules an action to be executed.
        /// </summary>
        /// <typeparam name="TState">The type of the state passed to the scheduled action.</typeparam>
        /// <param name="state">State passed to the action to be executed.</param>
        /// <param name="action">Action to be executed.</param>
        /// <returns>The disposable object used to cancel the scheduled action (best effort).</returns>
        /// <exception cref="ArgumentNullException"><paramref name="action"/> is null.</exception>
        public override IDisposable Schedule<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(action));

            var userWorkItem = new SystemReactive::System.Reactive.Concurrency.UserWorkItem<TState>(this, state, action);
            
            var res = ThreadPool.RunAsync(
                iaa => userWorkItem.Run(),
                Priority,
                Options);

            userWorkItem.CancelQueueDisposable = res.AsDisposable();

            return userWorkItem;
        }

        /// <summary>
        /// Schedules an action to be executed after dueTime, using a Windows.System.Threading.ThreadPoolTimer object.
        /// </summary>
        /// <typeparam name="TState">The type of the state passed to the scheduled action.</typeparam>
        /// <param name="state">State passed to the action to be executed.</param>

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