dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onNextAsync')

Error message

Value cannot be null. (Parameter 'onNextAsync')

What it means

The AsyncObserver constructor requires concrete delegate implementations for onNextAsync, onErrorAsync, and onCompletedAsync; it null-checks each and throws ArgumentNullException naming the offending parameter. A null onNextAsync would make it impossible to forward incoming elements, so construction fails immediately (line 17).

Solutions

1. Supply a real next handler: ValueTask-returning lambda such as async x => await HandleAsync(x).
2. Use AsyncObserver.Create(onNextAsync, onErrorAsync, onCompletedAsync) or the SubscribeAsync extensions, which document the same null requirements.
3. Use the built-in no-op/empty observers (e.g. AsyncObserver.Empty<T>() style helpers) if you need an inert observer.
4. If onNext truly should do nothing, pass _ => default instead of null.

Example fix

// before
var observer = new AsyncObserver<int>(null, ex => default, () => default);
// after
var observer = new AsyncObserver<int>(x => { Console.WriteLine(x); return default; }, ex => default, () => default);

if (onNextAsync == null || onErrorAsync == null || onCompletedAsync == null)
    throw new ArgumentException("AsyncObserver requires non-null onNextAsync, onErrorAsync, and onCompletedAsync delegates.");
var observer = new AsyncObserver<T>(onNextAsync, onErrorAsync, onCompletedAsync);

try
{
    var observer = new AsyncObserver<T>(onNextAsync, onErrorAsync, onCompletedAsync);
}
catch (ArgumentNullException ex) when (ex.ParamName == "onNextAsync")
{
    observer = new AsyncObserver<T>(_ => default, onErrorAsync, onCompletedAsync);
}

When it happens

Trigger: new AsyncObserver<T>(null, onErrorAsync, onCompletedAsync) — constructing the observer directly with a null next-handler delegate.

Common situations: Building custom observers by hand instead of using the AsyncObserver.Create factory or SubscribeAsync extension helpers; generic factory code that passes through a caller-supplied (possibly null) delegate; refactors that changed a lambda to null intending to remove behavior.

Related errors

• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'context')
• Value cannot be null. (Parameter 'disposable')
• Value cannot be null. (Parameter 'begin')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/cfc8b6a0f33ff35b. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObserver.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.Threading.Tasks;

namespace System.Reactive
{
    public class AsyncObserver<T> : AsyncObserverBase<T>
    {
        private readonly Func<T, ValueTask> _onNextAsync;
        private readonly Func<Exception, ValueTask> _onErrorAsync;
        private readonly Func<ValueTask> _onCompletedAsync;

        public AsyncObserver(Func<T, ValueTask> onNextAsync, Func<Exception, ValueTask> onErrorAsync, Func<ValueTask> onCompletedAsync)
        {
            _onNextAsync = onNextAsync ?? throw new ArgumentNullException(nameof(onNextAsync));
            _onErrorAsync = onErrorAsync ?? throw new ArgumentNullException(nameof(onErrorAsync));
            _onCompletedAsync = onCompletedAsync ?? throw new ArgumentNullException(nameof(onCompletedAsync));
        }

        protected override ValueTask OnCompletedAsyncCore() => _onCompletedAsync();

        protected override ValueTask OnErrorAsyncCore(Exception error) => _onErrorAsync(error ?? throw new ArgumentNullException(nameof(error)));

        protected override ValueTask OnNextAsyncCore(T value) => _onNextAsync(value);
    }
}

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