dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onError')

Error message

Value cannot be null. (Parameter 'onError')

What it means

The AnonymousObserver constructor throws ArgumentNullException when the onError action is null. Rx requires an explicit error handler so exceptions from the sequence are observed rather than silently swallowed; passing null onError is rejected at construction.

Solutions

1. Supply a real onError action, e.g. ex => Console.Error.WriteLine(ex)
2. If errors can be ignored deliberately, pass a no-op: _ => { }
3. Configure/default the error handler before subscribing

Example fix

// before
var sub = source.Subscribe(x => Handle(x), null, () => Done());
// after
var sub = source.Subscribe(x => Handle(x), ex => Log(ex), () => Done());

// C#
Action<Exception>? onError = ResolveErrorHandler();
if (onError is null)
    onError = ex => Console.Error.WriteLine(ex);
var sub = source.Subscribe(x => Handle(x), onError, () => Done());

bool HasErrorHandler(Action<Exception>? a) => a is not null;

try { var sub = source.Subscribe(onNext, onError, onCompleted); }
catch (ArgumentNullException ex) when (ex.ParamName == "onError")
{
    // supply a default error logger
}

When it happens

Trigger: Calling new AnonymousObserver<T>(onNext, null, onCompleted) or source.Subscribe(onNext, onError: null, onCompleted), e.g. source.Subscribe(x => ..., null, () => ...) where a logging/error delegate variable is null.

Common situations: Logging integrations where the error logger is not configured; test scaffolding that only wires onNext and onCompleted; plugin systems where the error handler is optional but the API demands it.

Related errors

• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onCompleted')
• observer
• observer
• observer

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/9f91fcb4256f9f67. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/AnonymousObserver.cs:27

    /// </summary>
    /// <typeparam name="T">The type of the elements in the sequence.</typeparam>
    public sealed class AnonymousObserver<T> : ObserverBase<T>
    {
        private readonly Action<T> _onNext;
        private readonly Action<Exception> _onError;
        private readonly Action _onCompleted;

        /// <summary>
        /// Creates an observer from the specified <see cref="IObserver{T}.OnNext(T)"/>, <see cref="IObserver{T}.OnError(Exception)"/>, and <see cref="IObserver{T}.OnCompleted()"/> actions.
        /// </summary>
        /// <param name="onNext">Observer's <see cref="IObserver{T}.OnNext(T)"/> action implementation.</param>
        /// <param name="onError">Observer's <see cref="IObserver{T}.OnError(Exception)"/> action implementation.</param>
        /// <param name="onCompleted">Observer's <see cref="IObserver{T}.OnCompleted()"/> action implementation.</param>
        /// <exception cref="ArgumentNullException"><paramref name="onNext"/> or <paramref name="onError"/> or <paramref name="onCompleted"/> is <c>null</c>.</exception>
        public AnonymousObserver(Action<T> onNext, Action<Exception> onError, Action onCompleted)
        {
            _onNext = onNext ?? throw new ArgumentNullException(nameof(onNext));
            _onError = onError ?? throw new ArgumentNullException(nameof(onError));
            _onCompleted = onCompleted ?? throw new ArgumentNullException(nameof(onCompleted));
        }

        /// <summary>
        /// Creates an observer from the specified <see cref="IObserver{T}.OnNext(T)"/> action.
        /// </summary>
        /// <param name="onNext">Observer's <see cref="IObserver{T}.OnNext(T)"/> action implementation.</param>
        /// <exception cref="ArgumentNullException"><paramref name="onNext"/> is <c>null</c>.</exception>
        public AnonymousObserver(Action<T> onNext)
            : this(onNext, Stubs.Throw, Stubs.Nop)
        {
        }

        /// <summary>
        /// Creates an observer from the specified <see cref="IObserver{T}.OnNext(T)"/> and <see cref="IObserver{T}.OnError(Exception)"/> actions.
        /// </summary>
        /// <param name="onNext">Observer's <see cref="IObserver{T}.OnNext(T)"/> action implementation.</param>
        /// <param name="onError">Observer's <see cref="IObserver{T}.OnError(Exception)"/> action implementation.</param>

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