dotnet/reactive · error · ArgumentNullException

nameof(source)

Error message

nameof(source)

What it means

The message is "Value cannot be null. (Parameter 'source')" thrown at Observable.Blocking.cs:563: Wait<TSource> validates that the IObservable<TSource> is non-null before blocking the calling thread until the sequence emits its single value. A null observable cannot be subscribed to, and Wait's contract (block and return the last element, throwing InvalidOperationException if empty) only makes sense on a real sequence.

Solutions

1. Null-check before waiting: if (obs != null) { var v = obs.Wait(); }
2. Fix the producer to return Observable.Empty<TSource>() (Wait will then surface the documented empty-sequence InvalidOperationException) or a real sequence instead of null.
3. Ensure initialization ordering: the observable-producing component must be initialized before any Wait call.

Example fix

// before
var result = _results.Wait(); // _results is null during startup
// after
if (_results == null)
{
    throw new InvalidOperationException("Results pipeline not initialized");
}
var result = _results.Wait();

if (source is null) throw new ArgumentNullException(nameof(source));
var result = source.Wait();

bool CanWait<T>(IObservable<T>? source) => source is not null;

try
{
    var result = source.Wait();
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    // source was null; handle or rethrow with context
    throw new InvalidOperationException("Result sequence not initialized", ex);
}

When it happens

Trigger: Calling Observable.Wait<TSource>(null) — e.g. waiting on a field/property holding the observable before it has been assigned, or on the result of an API that returned null instead of a sequence.

Common situations: Test code waiting on a subject/task-observable that failed to initialize; bridging synchronous code to Rx where the observable is produced by another component that returned null on failure; startup ordering bugs where Wait runs before the producer pipeline is built.

Related errors

• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'second')
• Value cannot be null. (Parameter 'sources')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/856d73b2f98c305d. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Blocking.cs:563

        #endregion

        #region + Wait +

        /// <summary>
        /// Waits for the observable sequence to complete and returns the last element of the sequence.
        /// If the sequence terminates with an OnError notification, the exception is thrown.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Source observable sequence.</param>
        /// <returns>The last element in the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        /// <exception cref="InvalidOperationException">The source sequence is empty.</exception>
        public static TSource Wait<TSource>(this IObservable<TSource> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            return s_impl.Wait(source);
        }

        #endregion
    }
}

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