dotnet/reactive · error · ArgumentNullException

addHandler

Error message

addHandler

What it means

The non-generic, non-converting FromEvent<TEventArgs>(addHandler, removeHandler) in FromEvent.cs null-checks addHandler first. It throws ArgumentNullException with parameter name 'addHandler' when the subscription action is null. addHandler attaches an Action<TEventArgs> to the underlying event; without it the operator cannot subscribe, so it fails fast before returning the observable.

Solutions

1. Pass h => source.Event += h as addHandler.
2. Ensure the argument is produced before the FromEvent call, not lazily.
3. Use the conversion-based overloads when the handler type needs adapting.
4. Add a unit test asserting non-null arguments when building event wrappers.

Example fix

// before
var obs = AsyncObservable.FromEvent<EventArgs>(null, h => obj.MyEvent -= h);
// after
var obs = AsyncObservable.FromEvent<EventArgs>(h => obj.MyEvent += h, h => obj.MyEvent -= h);

if (addHandler is null) throw new ArgumentNullException(nameof(addHandler));
if (removeHandler is null) throw new ArgumentNullException(nameof(removeHandler));

static bool Valid<TEventArgs>(Action<Action<TEventArgs>>? add, Action<Action<TEventArgs>>? remove) => add != null && remove != null;

try { var obs = AsyncObservable.FromEvent<EventArgs>(add, remove); } catch (ArgumentNullException ex) { log.LogError(ex, "FromEvent requires non-null {Param}", ex.ParamName); }

When it happens

Trigger: Calling AsyncObservable.FromEvent<TEventArgs>(addHandler, removeHandler) with addHandler == null; FromEvent.cs:17 is that `throw new ArgumentNullException(nameof(addHandler))`.

Common situations: Wrapping classic events like button.Click where the add lambda was omitted; passing a nullable delegate field not yet assigned; generic helper that takes add/remove and receives null from its caller.

Related errors

• removeHandler
• Value cannot be null. (Parameter 'observer')
• ArgumentNull_Generic
• conversion
• Value cannot be null. (Parameter 'action')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/cd86d8ef300adb57. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEvent.cs:17

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Reactive.Concurrency;
using System.Reactive.Disposables;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TEventArgs> FromEvent<TEventArgs>(Action<Action<TEventArgs>> addHandler, Action<Action<TEventArgs>> removeHandler)
        {
            if (addHandler == null)
                throw new ArgumentNullException(nameof(addHandler));
            if (removeHandler == null)
                throw new ArgumentNullException(nameof(removeHandler));

            return FromEventCore<Action<TEventArgs>, TEventArgs>(h => h, addHandler, removeHandler, GetSchedulerForCurrentContext());
        }

        public static IAsyncObservable<TEventArgs> FromEvent<TEventArgs>(Action<Action<TEventArgs>> addHandler, Action<Action<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));

            return FromEventCore<Action<TEventArgs>, TEventArgs>(h => h, addHandler, removeHandler, scheduler);
        }

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