dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'onCompletedAsync')

Error message

Value cannot be null. (Parameter 'onCompletedAsync')

What it means

The AsyncObserver constructor null-checks all three delegates; onCompletedAsync was null, so it throws ArgumentNullException. Without a completion delegate the observer could not signal graceful end-of-stream, so construction fails at line 19.

Solutions

1. Supply a completion handler, e.g. () => { Console.WriteLine("completed"); return default; } or a no-op () => default.
2. Use AsyncObserver.Create or the SubscribeAsync extension overloads instead of constructing AsyncObserver directly.
3. Model truly-never-completing sequences with a no-op delegate rather than null.
4. Centralize observer creation in one helper that supplies default no-op delegates so nulls never reach the constructor.

Example fix

// before
var observer = new AsyncObserver<int>(x => default, ex => default, null);
// after
var observer = new AsyncObserver<int>(x => 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 == "onCompletedAsync")
{
    observer = new AsyncObserver<T>(onNextAsync, onErrorAsync, () => default);
}

When it happens

Trigger: new AsyncObserver<T>(onNextAsync, onErrorAsync, null) — constructing with a null completion delegate.

Common situations: Assuming completion is optional for infinite streams and passing null; test scaffolding that fills in next/error but not completed; factory code plumbing through user-supplied delegates without defaults.

Related errors

• Value cannot be null. (Parameter 'onNextAsync')
• Value cannot be null. (Parameter 'onErrorAsync')
• 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/67e666177826b1ba. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/AsyncObserver.cs:19

// 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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