dotnet/reactive · error · ArgumentNullException

source

Error message

source

What it means

The ListObservable constructor subscribes to the given source and throws ArgumentNullException when 'source' is null, since the observable to record is required to build the internal results list and subject.

Solutions

1. Pass a valid IObservable<T> instance (use Observable.Never<T>() or Empty as a placeholder if needed)
2. Null-check the source before constructing ListObservable
3. Fix the factory/dependency that returned null

Example fix

// before
var obs = new ListObservable<int>(maybeSource); // may be null
// after
if (maybeSource == null) maybeSource = Observable.Empty<int>();
var obs = new ListObservable<int>(maybeSource);

if (source == null) throw new ArgumentNullException(nameof(source));
var obs = new ListObservable<T>(source);

static bool IsObservable<T>(IObservable<T> s) => s != null;

try { var obs = new ListObservable<int>(source); } catch (ArgumentNullException) { /* null source */ }

When it happens

Trigger: Constructing new ListObservable<T>(null), e.g. passing an uninitialized observable field or a factory result that was null.

Common situations: Test harness code capturing notifications from a stream whose setup failed silently; DI/wiring produced null for the source observable.

Related errors

• Value cannot be null. (Parameter 'comparer')
• observer
• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/561f0305da47ca4a. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/ListObservable.cs:35

    /// </summary>
    /// <typeparam name="T">The type of elements received from the source sequence.</typeparam>
    [Experimental]
    public class ListObservable<T> : IList<T>, IObservable<object>
    {
        private readonly IDisposable _subscription;
        private readonly AsyncSubject<object> _subject = new();
        private readonly List<T> _results = [];

        /// <summary>
        /// Constructs an object that retains the values of source and signals the end of the sequence.
        /// </summary>
        /// <param name="source">The observable sequence whose elements will be retained in the list.</param>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is <c>null</c>.</exception>
        public ListObservable(IObservable<T> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            _subscription = source.Subscribe(_results.Add, _subject.OnError, _subject.OnCompleted);
        }

        private void Wait()
        {
            _subject.DefaultIfEmpty().Wait();
        }

        /// <summary>
        /// Returns the last value of the observable sequence.
        /// </summary>
        public T Value
        {
            get
            {
                Wait();

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