dotnet/reactive · error · ArgumentNullException

source

Error message

source

What it means

ControlObservable.SubscribeOn is an extension method that marshals subscription of the source sequence onto the Windows Forms control's message loop via a ControlScheduler. It validates both arguments and throws ArgumentNullException naming 'source' when the observable is null. The library cannot wrap a null sequence.

Solutions

1. Ensure the source IObservable<TSource> is non-null before calling SubscribeOn.
2. Fix the producer that is returning null instead of a sequence (use Observable.Empty or Observable.Throw).
3. Guard with a null check or coalesce to an empty sequence before subscribing.

Example fix

// before
var sub = maybeSource.SubscribeOn(this.pictureBox);
// after
var sub = (maybeSource ?? Observable.Empty<Image>()).SubscribeOn(this.pictureBox);

if (source == null) source = Observable.Empty<TSource>();
if (control == null) throw new InvalidOperationException("Control not initialized");
var sub = source.SubscribeOn(control);

static bool CanSubscribeOn<TSource>(IObservable<TSource> source, Control control) => source is not null && control is not null;

try { var sub = source.SubscribeOn(control); }
catch (ArgumentNullException ex) when (ex.ParamName is "source" or "control") { log.Error($"SubscribeOn got null {ex.ParamName}"); }

When it happens

Trigger: Calling nullSource.SubscribeOn(someControl), often when the observable comes from a method that returned null, a failed service call, or an uninitialized field.

Common situations: UI data-binding code where the observable-producing service returns null on error; refactors where a reactive property was never initialized; WinForms screens composed before the data source is ready.

Related errors

• control
• source
• dispatcher
• dispatcher (Parameter 'dispatcher')
• Value cannot be null. (Parameter 'subscribeAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2434c38eba5d87d0. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Platforms/Desktop/Linq/ControlObservable.cs:31

    public static class ControlObservable
    {
        /// <summary>
        /// Wraps the source sequence in order to run its subscription and unsubscription logic on the Windows Forms message loop associated with the specified control.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="control">Windows Forms control whose associated message loop is used to perform subscription and unsubscription actions on.</param>
        /// <returns>The source sequence whose subscriptions and unsubscriptions happen on the Windows Forms message loop associated with the specified control.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="control"/> is null.</exception>
        /// <remarks>
        /// Only the side-effects of subscribing to the source sequence and disposing subscriptions to the source sequence are run on the specified control.
        /// In order to invoke observer callbacks on the specified control, e.g. to render results in a control, use <see cref="ObserveOn"/>.
        /// </remarks>
        public static IObservable<TSource> SubscribeOn<TSource>(this IObservable<TSource> source, Control control)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if (control == null)
            {
                throw new ArgumentNullException(nameof(control));
            }

            return Synchronization.SubscribeOn(source, new ControlScheduler(control));
        }

        /// <summary>
        /// Wraps the source sequence in order to run its observer callbacks on the Windows Forms message loop associated with the specified control.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="control">Windows Forms control whose associated message loop is used to notify observers on.</param>
        /// <returns>The source sequence whose observations happen on the Windows Forms message loop associated with the specified control.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="control"/> is null.</exception>

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