dotnet/reactive · error · ArgumentNullException

nameof(scheduler)

Error message

nameof(scheduler)

What it means

System.ArgumentNullException with param name 'scheduler' (message records nameof(scheduler)). The conversion-based overload with an explicit scheduler, FromEventPattern<TDelegate, TEventArgs>(conversion, addHandler, removeHandler, scheduler), ends its validation chain with a null check on scheduler and throws ArgumentNullException when it is null. Every other argument may be fine; only the scheduler is at fault.

Solutions

1. Pass a valid IAsyncScheduler (AsyncScheduler.Default, TestScheduler, DI-resolved instance).
2. Use the three-parameter conversion overload without scheduler to get the context scheduler automatically.
3. Fix scheduler initialization/registration so the referenced value is non-null.
4. Assert the scheduler before the call to fail at the right place.

Example fix

// before
var obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(
    h => h,
    x => target.E += x,
    x => target.E -= x,
    null);
// after
var obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(
    h => h,
    x => target.E += x,
    x => target.E -= x,
    AsyncScheduler.Default);

if (scheduler == null) throw new ArgumentNullException(nameof(scheduler)); // or omit to use context scheduler

static bool ValidConversionArgs<TDelegate>(Func<EventHandler<TEventArgs>, TDelegate> conv, Action<TDelegate> add, Action<TDelegate> remove, IAsyncScheduler s)
    => conv != null && add != null && remove != null && s != null;

try
{
    var obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(conversion, add, remove, scheduler);
}
catch (ArgumentNullException ex) when (ex.ParamName == "scheduler")
{
    // scheduler null; default or fix initialization
}

When it happens

Trigger: Calling the four-parameter conversion overload with null as the last IAsyncScheduler argument, or a scheduler variable resolved lazily from DI/config that is null at call time.

Common situations: Test setups where the scheduler field is not assigned, configuration that yields no scheduler, or an overload mix-up appending null where the parameterless-scheduler variant was intended.

Related errors

• scheduler
• Value cannot be null. (Parameter 'scheduler')
• removeHandler
• conversion
• nameof(addHandler)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/aa0c8c3d6d3e83ba. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs:59

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

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

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

        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TDelegate, TEventArgs>(Func<EventHandler<TEventArgs>, TDelegate> conversion, Action<TDelegate> addHandler, Action<TDelegate> removeHandler, IAsyncScheduler scheduler)
        {
            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));
        }

View on GitHub (pinned to 94b5d5ab91)