dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'messages')

Error message

Value cannot be null. (Parameter 'messages')

What it means

TestScheduler.CreateHotObservable throws ArgumentNullException when the messages array parameter is null. It builds a HotObservable that replays Recorded<Notification<T>> marble messages; a null message list cannot represent a valid sequence, so it is rejected before construction. Note an empty array is allowed (a hot observable that never emits).

Solutions

1. Pass at least an empty array instead of null: CreateHotObservable<int>()
2. Fix the array-producing helper to return Array.Empty<Recorded<Notification<int>>>() when there are no messages
3. Null-check parsed marble input before constructing the observable

Example fix

// before
var xs = scheduler.CreateHotObservable<int>(ParseMarbles(null)); // null array
// after
var msgs = ParseMarbles(marbleString) ?? Array.Empty<Recorded<Notification<int>>>();
var xs = scheduler.CreateHotObservable(msgs);

// C#
Recorded<Notification<int>>[]? msgs = BuildMessages(marble);
if (msgs is null)
    msgs = Array.Empty<Recorded<Notification<int>>>();
var xs = scheduler.CreateHotObservable(msgs);

bool HasMessages<T>(Recorded<Notification<T>>[]? m) => m is not null;

try { var xs = scheduler.CreateHotObservable(msgs); }
catch (ArgumentNullException ex) when (ex.ParamName == "messages")
{
    xs = scheduler.CreateHotObservable<T>(); // empty hot observable
}

When it happens

Trigger: Calling scheduler.CreateHotObservable<T>(null) explicitly, or passing a Recorded<Notification<T>>[] variable that is null (e.g. from a failed lookup or deserialization), while an empty params list is fine.

Common situations: Marble-test helpers that build message arrays dynamically and return null on parse failure; data-driven tests with null entries; refactors converting marble-string parsers to array builders that leave null results.

Related errors

• action
• action (Parameter 'action')
• action (Parameter 'action')
• action (Value cannot be null)
• addHandler

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/f1164244a445a050. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/Microsoft.Reactive.Testing/TestScheduler.cs:161

            {
                throw new ArgumentNullException(nameof(create));
            }

            return Start(create, ReactiveTest.Created, ReactiveTest.Subscribed, ReactiveTest.Disposed);
        }

        /// <summary>
        /// Creates a hot observable using the specified timestamped notification messages.
        /// </summary>
        /// <typeparam name="T">The element type of the observable sequence being created.</typeparam>
        /// <param name="messages">Notifications to surface through the created sequence at their specified absolute virtual times.</param>
        /// <returns>Hot observable sequence that can be used to assert the timing of subscriptions and notifications.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="messages"/> is null.</exception>
        public ITestableObservable<T> CreateHotObservable<T>(params Recorded<Notification<T>>[] messages)
        {
            if (messages == null)
            {
                throw new ArgumentNullException(nameof(messages));
            }

            return new HotObservable<T>(this, messages);
        }

        /// <summary>
        /// Creates a cold observable using the specified timestamped notification messages.
        /// </summary>
        /// <typeparam name="T">The element type of the observable sequence being created.</typeparam>
        /// <param name="messages">Notifications to surface through the created sequence at their specified virtual time offsets from the sequence subscription time.</param>
        /// <returns>Cold observable sequence that can be used to assert the timing of subscriptions and notifications.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="messages"/> is null.</exception>
        public ITestableObservable<T> CreateColdObservable<T>(params Recorded<Notification<T>>[] messages)
        {
            if (messages == null)
            {
                throw new ArgumentNullException(nameof(messages));
            }

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