dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'scheduler')

Error message

Value cannot be null. (Parameter 'scheduler')

What it means

ScheduledDisposable's constructor requires a non-null IScheduler on which the wrapped resource will be disposed; null is rejected with ArgumentNullException. The scheduler is stored in a get-only property and is essential for DisposeAsync/Dispose routing.

Solutions

1. Pass an actual scheduler instance, e.g. Scheduler.Default or Scheduler.Immediate.
2. Resolve/register IScheduler in your DI container before constructing the ScheduledDisposable.
3. Fall back to Scheduler.Immediate when no scheduler is configured and immediate disposal is acceptable.

Example fix

// before
var sd = new ScheduledDisposable(configuredScheduler, resource); // configuredScheduler is null
// after
var sd = new ScheduledDisposable(configuredScheduler ?? Scheduler.Default, resource);

if (scheduler is null) scheduler = Scheduler.Default;
var sd = new ScheduledDisposable(scheduler, resource);

static bool HasScheduler(IScheduler s) => s is not null;

try { var sd = new ScheduledDisposable(scheduler, resource); }
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler") { /* use Scheduler.Default fallback */ }

When it happens

Trigger: Calling new ScheduledDisposable(null, someDisposable) — passing null for the 'scheduler' parameter, e.g. when a scheduler variable or DI-resolved IScheduler is null.

Common situations: DI container that failed to register IScheduler, configuration where the scheduler is chosen conditionally and both branches yield null, or testing code with a stubbed scheduler left unassigned.

Related errors

• Value cannot be null. (Parameter 'scheduler')
• Value cannot be null. (Parameter 'scheduler')
• Value cannot be null. (Parameter 'disposable')
• Value cannot be null. (Parameter 'disposable')
• throw new ArgumentNullException(nameof(scheduler));

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2d9cacf0cd6d9b54. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Disposables/ScheduledDisposable.cs:24

namespace System.Reactive.Disposables
{
    /// <summary>
    /// Represents a disposable resource whose disposal invocation will be scheduled on the specified <seealso cref="IScheduler"/>.
    /// </summary>
    public sealed class ScheduledDisposable : ICancelable
    {
        private SingleAssignmentDisposableValue _disposable;

        /// <summary>
        /// Initializes a new instance of the <see cref="ScheduledDisposable"/> class that uses an <see cref="IScheduler"/> on which to dispose the disposable.
        /// </summary>
        /// <param name="scheduler">Scheduler where the disposable resource will be disposed on.</param>
        /// <param name="disposable">Disposable resource to dispose on the given scheduler.</param>
        /// <exception cref="ArgumentNullException"><paramref name="scheduler"/> or <paramref name="disposable"/> is null.</exception>
        public ScheduledDisposable(IScheduler scheduler, IDisposable disposable)
        {
            Scheduler = scheduler ?? throw new ArgumentNullException(nameof(scheduler));
            _disposable.Disposable = disposable ?? throw new ArgumentNullException(nameof(disposable));
        }

        /// <summary>
        /// Gets the scheduler where the disposable resource will be disposed on.
        /// </summary>
        public IScheduler Scheduler { get; }

        /// <summary>
        /// Gets the underlying disposable. After disposal, the result is undefined.
        /// </summary>
        public IDisposable Disposable => _disposable.Disposable ?? Disposables.Disposable.Empty;

        /// <summary>
        /// Gets a value that indicates whether the object is disposed.
        /// </summary>
        public bool IsDisposed => _disposable.IsDisposed;

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