dotnet/reactive · error · ArgumentNullException

nameof(addHandler)

Error message

nameof(addHandler)

What it means

System.ArgumentNullException with param name 'addHandler' (message records nameof(addHandler)). In the two-sender overload FromEventPattern<TDelegate, TSender, TEventArgs>(addHandler, removeHandler[, scheduler]) the first validation throws when addHandler is null; the FromEvent operator needs it to attach the converted handler that yields (sender, args) pairs.

Solutions

1. Pass the attach delegate first: x => target.E += x.
2. Check argument order: addHandler precedes removeHandler in every overload.
3. Ensure dynamically created add delegates are non-null before calling.
4. Validate arguments at the call site with a clear assertion.

Example fix

// before
var obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(
    null,
    x => target.E -= x);
// after
var obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(
    x => target.E += x,
    x => target.E -= x);

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

static bool ValidSenderArgs<TDelegate>(Action<TDelegate> add, Action<TDelegate> remove) => add != null && remove != null;

try
{
    var obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(add, remove);
}
catch (ArgumentNullException ex) when (ex.ParamName == "addHandler")
{
    // first lambda was null; fix argument order or wiring
}

When it happens

Trigger: Calling FromEventPattern<TDelegate, TSender, TEventArgs>(null, removeHandler) or the scheduler variant with a null first Action<TDelegate>, e.g. misordered arguments or a reflection-built attach delegate that is null.

Common situations: Argument order mistakes (remove lambda passed first), reflection-based wiring failures producing a null delegate, or conditional wiring where the attach step is skipped for some events.

Related errors

• removeHandler
• scheduler
• conversion
• nameof(scheduler)
• conversion

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/653a5414fe89b4cd. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs:69

        {
            if (conversion == null)
                throw new ArgumentNullException(nameof(conversion));
            if (addHandler == null)
                throw new ArgumentNullException(nameof(addHandler));
            if (removeHandler == null)
                throw new ArgumentNullException(nameof(removeHandler));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return FromEvent<TDelegate, object, TEventArgs>(action => conversion(new EventHandler<TEventArgs>((o, e) => action(o, e))), addHandler, removeHandler, scheduler).Select(t => new EventPattern<TEventArgs>(t.arg1, t.arg2));
        }

        public static IAsyncObservable<EventPattern<TSender, TEventArgs>> FromEventPattern<TDelegate, TSender, TEventArgs>(Action<TDelegate> addHandler, Action<TDelegate> removeHandler) => FromEventPattern<TDelegate, TSender, TEventArgs>(addHandler, removeHandler, GetSchedulerForCurrentContext());

        public static IAsyncObservable<EventPattern<TSender, TEventArgs>> FromEventPattern<TDelegate, TSender, TEventArgs>(Action<TDelegate> addHandler, Action<TDelegate> removeHandler, IAsyncScheduler scheduler)
        {
            if (addHandler == null)
                throw new ArgumentNullException(nameof(addHandler));
            if (removeHandler == null)
                throw new ArgumentNullException(nameof(removeHandler));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

            return FromEvent<TDelegate, TSender, TEventArgs>(action => ConvertDelegate<Action<TSender, TEventArgs>, TDelegate>(action), addHandler, removeHandler, scheduler).Select(t => new EventPattern<TSender, TEventArgs>(t.arg1, t.arg2));
        }

        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Action<EventHandler<TEventArgs>> addHandler, Action<EventHandler<TEventArgs>> removeHandler) => FromEventPattern<TEventArgs>(addHandler, removeHandler, GetSchedulerForCurrentContext());

        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Action<EventHandler<TEventArgs>> addHandler, Action<EventHandler<TEventArgs>> removeHandler, IAsyncScheduler scheduler)
        {
            if (addHandler == null)
                throw new ArgumentNullException(nameof(addHandler));
            if (removeHandler == null)
                throw new ArgumentNullException(nameof(removeHandler));
            if (scheduler == null)
                throw new ArgumentNullException(nameof(scheduler));

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