dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'leftDurationSelector')

Error message

Value cannot be null. (Parameter 'leftDurationSelector')

What it means

GroupJoin threw ArgumentNullException because the leftDurationSelector delegate was null. The selector maps each left element to the observable defining its duration window; the public operator validates it synchronously.

Solutions

1. Supply a non-null duration selector, e.g. _ => Observable.Timer(TimeSpan.FromSeconds(5)) or _ => Observable.Empty<Unit>() for zero-duration.
2. Null-check the delegate before the call and fail with context.
3. Use Observable.Never<Unit>() if left elements should stay open indefinitely.

Example fix

// before
var joined = left.GroupJoin(right, null, r => r.Window(), (l, rs) => ...);
// after
var joined = left.GroupJoin(right, _ => Observable.Timer(TimeSpan.FromSeconds(5)), r => r.Window(), (l, rs) => ...);

if (leftDurationSelector == null)
    leftDurationSelector = _ => Observable.Never<Unit>();

bool HasLeftDuration<TLeft>(Func<TLeft, IObservable<Unit>>? f) => f is not null;

try
{
    var joined = left.GroupJoin(right, leftDurationSelector, rightDurationSelector, resultSelector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "leftDurationSelector")
{
    // use default window policy
}

When it happens

Trigger: Calling left.GroupJoin(right, null, rightDurationSelector, resultSelector) — e.g. a window selector built conditionally that ended up null.

Common situations: Conditional window construction; forwarding optional method parameters; forgetting to replace a placeholder null after refactoring the join call.

Related errors

• Value cannot be null. (Parameter 'left')
• Value cannot be null. (Parameter 'resultSelector')
• Value cannot be null. (Parameter 'right')
• Value cannot be null. (Parameter 'rightDurationSelector')
• Value cannot be null. (Parameter 'onCompleted')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c4efd42255cd2f21. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.StandardSequenceOperators.cs:856

        /// <param name="rightDurationSelector">A function to select the duration of each element of the right observable sequence, used to determine overlap.</param>
        /// <param name="resultSelector">A function invoked to compute a result element for any element of the left sequence with overlapping elements from the right observable sequence.</param>
        /// <returns>An observable sequence that contains result elements computed from source elements that have an overlapping duration.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="left"/> or <paramref name="right"/> or <paramref name="leftDurationSelector"/> or <paramref name="rightDurationSelector"/> or <paramref name="resultSelector"/> is null.</exception>
        public static IObservable<TResult> GroupJoin<TLeft, TRight, TLeftDuration, TRightDuration, TResult>(this IObservable<TLeft> left, IObservable<TRight> right, Func<TLeft, IObservable<TLeftDuration>> leftDurationSelector, Func<TRight, IObservable<TRightDuration>> rightDurationSelector, Func<TLeft, IObservable<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 s_impl.GroupJoin(left, right, leftDurationSelector, rightDurationSelector, resultSelector);
        }

        #endregion

        #region + Join +

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