dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source')

Error message

Value cannot be null. (Parameter 'source')

What it means

The protected EventPatternSourceBase constructor requires a non-null source sequence. Passing null throws ArgumentNullException for 'source'. This base class backs FromEventPattern-style bridges that expose an observable as a .NET event, so a source is mandatory.

Solutions

1. Pass a valid non-null observable, e.g. Observable.FromEventPattern<...>(...) result.
2. Null-check or ?? Observable.Empty<EventPattern<TSender,TEventArgs>>() the value before calling the base constructor.

Example fix

// before
base(source, invokeHandler)
// after
base(source ?? Observable.Empty<EventPattern<TSender, TEventArgs>>(), invokeHandler)

if (source == null) throw new ArgumentNullException(nameof(source));

bool IsValidSource<TSender, TArgs>(IObservable<EventPattern<TSender, TArgs>>? s) => s is not null;

try { new MyEventPatternSource(source, invoke); } catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* supply fallback observable */ }

When it happens

Trigger: A derived class (or user code subclassing EventPatternSourceBase) passing null for the IObservable<EventPattern<TSender,TEventArgs>> source parameter.

Common situations: Custom event-pattern adapters where the observable is produced by a factory that returned null; refactorings that dropped a null-coalescing operator on the source.

Related errors

• Value cannot be null. (Parameter 'invokeHandler')
• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/fb976ec37bc48055. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/EventPatternSourceBase.cs:90

                        _isAdded = true;
                    }
                }
            }
        }

        private readonly IObservable<EventPattern<TSender, TEventArgs>> _source;
        private readonly Dictionary<Delegate, Stack<IDisposable>> _subscriptions = [];
        private readonly Action<Action<TSender?, TEventArgs>, /*object,*/ EventPattern<TSender, TEventArgs>> _invokeHandler;

        /// <summary>
        /// Creates a new event pattern source.
        /// </summary>
        /// <param name="source">Source sequence to expose as an event.</param>
        /// <param name="invokeHandler">Delegate used to invoke the event for each element of the sequence.</param>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="invokeHandler"/> is <c>null</c>.</exception>
        protected EventPatternSourceBase(IObservable<EventPattern<TSender, TEventArgs>> source, Action<Action<TSender?, TEventArgs>, /*object,*/ EventPattern<TSender, TEventArgs>> invokeHandler)
        {
            _source = source ?? throw new ArgumentNullException(nameof(source));
            _invokeHandler = invokeHandler ?? throw new ArgumentNullException(nameof(invokeHandler));
        }

        /// <summary>
        /// Adds the specified event handler, causing a subscription to the underlying source.
        /// </summary>
        /// <param name="handler">Event handler to add. The same delegate should be passed to the <see cref="Remove(Delegate)"/> operation in order to remove the event handler.</param>
        /// <param name="invoke">Invocation delegate to raise the event in the derived class.</param>
        /// <exception cref="ArgumentNullException"><paramref name="handler"/> or <paramref name="invoke"/> is <c>null</c>.</exception>
        protected void Add(Delegate handler, Action<TSender?, TEventArgs> invoke)
        {
            if (handler == null)
            {
                throw new ArgumentNullException(nameof(handler));
            }

            if (invoke == null)
            {

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