dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'right')

Error message

Value cannot be null. (Parameter 'right')

What it means

GroupJoin requires the `right` source sequence; a null right-hand observable is rejected with ArgumentNullException as the second guard in the method. Without a right sequence there is nothing to join durations against, so fail-fast is intentional.

Solutions

1. Substitute AsyncObservable.Empty<TRight>() when the right stream is intentionally absent.
2. Null-check/initialize the right observable before composing the join.
3. Verify the factory method producing the right stream never returns null.

Example fix

// before
var result = left.GroupJoin(maybeRight, lds, rds, rs);
// after
var result = left.GroupJoin(maybeRight ?? AsyncObservable.Empty<TRight>(), lds, rds, rs);

if (right is null) throw new ArgumentNullException(nameof(right));
// or normalize:
right ??= AsyncObservable.Empty<TRight>();

bool IsValidSource<T>(IAsyncObservable<T>? s) => s is not null;

try
{
    var result = left.GroupJoin(right, lds, rds, rs);
}
catch (ArgumentNullException ex) when (ex.ParamName == "right")
{
    var result = AsyncObservable.Empty<TResult>();
}

When it happens

Trigger: Calling `left.GroupJoin(null, leftDurationSelector, rightDurationSelector, resultSelector)` or passing a null-returning right-stream factory to the static AsyncObservable.GroupJoin.

Common situations: Right stream conditionally loaded (e.g. null when a feed is disabled); config-driven stream selection returning null; refactoring that renamed the right-source variable ahead of its assignment.

Related errors

• leftDurationSelector
• rightDurationSelector
• Value cannot be null. (Parameter 'left')
• Argument cannot be null (Parameter name: functionAsync)
• Argument cannot be null (Parameter name: observer)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/51be17d8d29d992c. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/GroupJoin.cs:20

// 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.Collections.Generic;
using System.Reactive.Disposables;
using System.Reactive.Subjects;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TResult> GroupJoin<TLeft, TRight, TLeftDuration, TRightDuration, TResult>(this IAsyncObservable<TLeft> left, IAsyncObservable<TRight> right, Func<TLeft, IAsyncObservable<TLeftDuration>> leftDurationSelector, Func<TRight, IAsyncObservable<TRightDuration>> rightDurationSelector, Func<TLeft, IAsyncObservable<TRight>, TResult> resultSelector)
        {
            if (left == null)
                throw new ArgumentNullException(nameof(left));
            if (right == null)
                throw new ArgumentNullException(nameof(right));
            if (leftDurationSelector == null)
                throw new ArgumentNullException(nameof(leftDurationSelector));
            if (rightDurationSelector == null)
                throw new ArgumentNullException(nameof(rightDurationSelector));
            if (resultSelector == null)
                throw new ArgumentNullException(nameof(resultSelector));

            return Create<TResult>(async observer =>
            {
                var subscriptions = new CompositeAsyncDisposable();

                var (leftObserver, rightObserver, disposable) = AsyncObserver.GroupJoin(observer, subscriptions, leftDurationSelector, rightDurationSelector, resultSelector);

                var leftSubscription = await left.SubscribeSafeAsync(leftObserver).ConfigureAwait(false);
                await subscriptions.AddAsync(leftSubscription).ConfigureAwait(false);

                var rightSubscription = await right.SubscribeSafeAsync(rightObserver).ConfigureAwait(false);
                await subscriptions.AddAsync(rightSubscription).ConfigureAwait(false);

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