What causes "Value cannot be null. (Parameter 'invokeHandler')" in dotnet/reactive?

Supply a valid delegate that invokes the event handler, e.g. (h, e) => handlerTarget?.Invoke(e.Sender, e.EventArgs).

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'invokeHandler')

Error message

Value cannot be null. (Parameter 'invokeHandler')

What it means

The protected EventPatternSourceBase constructor also requires a non-null invokeHandler delegate used to raise the event for each element. Passing null throws ArgumentNullException for 'invokeHandler'.

Solutions

Supply a valid delegate that invokes the event handler, e.g. (h, e) => handlerTarget?.Invoke(e.Sender, e.EventArgs).
Null-check the delegate before constructing the EventPatternSourceBase.

Example fix

// before
new MyEventPatternSource(source, invokeHandler);
// after
if (invokeHandler == null) throw new InvalidOperationException("invokeHandler not wired");
new MyEventPatternSource(source, invokeHandler);

Defensive patterns

Strategy: validation

Validate before calling

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

Type guard

bool IsValidHandler<TSender, TArgs>(Action<Action<TSender?, TArgs>, EventPattern<TSender, TArgs>>? h) => h is not null;

Try / catch

try { new MyEventPatternSource(source, invokeHandler); } catch (ArgumentNullException ex) when (ex.ParamName == "invokeHandler") { /* wire the raise delegate */ }

Prevention

Define the event-raise lambda inline where the base constructor is called.
Never resolve invoke handlers from registries that can return null without checking.

When it happens

Trigger: Subclassing EventPatternSourceBase and passing null as the Action<Action<TSender?,TEventArgs>, EventPattern<TSender,TEventArgs>> invokeHandler argument.

Common situations: Custom FromEventPattern-like adapters where the delegate wiring the event raise is conditionally assigned or resolved from a registry that returned null.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c44d4a861494e481. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/EventPatternSourceBase.cs:91

                    }
                }
            }
        }

        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)
            {
                throw new ArgumentNullException(nameof(invoke));

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