dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'handler')

Error message

Value cannot be null. (Parameter 'handler')

What it means

The protected Add(Delegate handler, Action<TSender,TEventArgs> invoke) method registers an event handler and starts a subscription; both arguments are null-checked first. A null handler is rejected with ArgumentNullException because it is used as the dictionary key for later Remove.

Solutions

1. Pass a non-null Delegate as handler.
2. Add a null check in the public wrapper before calling protected Add so the caller gets a clearer error.
3. If the handler may be absent, skip the Add call instead of forwarding null.

Example fix

// before
public void AddHandler(EventHandler<EventArgs> h) => Add(h, (s, e) => h?.Invoke(s, e));
// after
public void AddHandler(EventHandler<EventArgs> h)
{
    if (h == null) throw new ArgumentNullException(nameof(h));
    Add(h, (s, e) => h(s, e));
}

if (handler is null) throw new ArgumentNullException(nameof(handler));
Add(handler, invoke);

static bool HasHandler(Delegate? d) => d is not null;

try { Add(handler, invoke); }
catch (ArgumentNullException ex) when (ex.ParamName == "handler")
{ log.LogWarning("cannot add a null event handler"); }

When it happens

Trigger: Calling Add(null, invoke) in a derived event-pattern class, e.g. forwarding an event-handler argument that was null from the public API surface.

Common situations: A public AddHandler(Handler h) forwarding h directly without checking; callers passing a lambda they expected to be wrapped; marshalled/UI event handlers that failed to resolve.

Related errors

• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'invokeHandler')
• Value cannot be null. (Parameter 'invoke')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'invokeHandler')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/1de26fe187042bae. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Internal/EventPatternSourceBaseInternal.cs:50

        /// <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 null.</exception>
        protected EventPatternSourceBaseInternal(IAsyncObservable<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));
            _subscriptions = new Dictionary<Delegate, Stack<IAsyncDisposable>>();
        }

        /// <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 Remove 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 null.</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));

            var gate = new object();
            var isAdded = false;
            var isDone = false;

            var remove = new Action(() =>
            {
                lock (gate)
                {
                    if (isAdded)
                        Remove(handler);
                    else
                        isDone = true;
                }
            });

View on GitHub (pinned to 94b5d5ab91)