dotnet/reactive · error · ArgumentNullException

keySelector

Error message

keySelector

What it means

AsyncObservable.GroupByUntil (2-argument overload) throws ArgumentNullException when the keySelector delegate is null. The key selector assigns each element to a group and is captured in the operator state, so it must be provided. This guard executes synchronously when the operator is constructed.

Solutions

1. Pass a concrete key selector such as x => x.GroupKey.
2. Validate delegates in your wrapper methods before forwarding.
3. Avoid optional key-selector parameters; require them in the signature.

Example fix

// before
public static IOpped Group<T>(IAsyncObservable<T> s, Func<T, string> k = null)
    => s.GroupByUntil(k, g => g.Take(1));
// after
public static IOpped Group<T>(IAsyncObservable<T> s, Func<T, string> k)
    => s.GroupByUntil(k, g => g.Take(1));

if (keySelector is null) throw new ArgumentNullException(nameof(keySelector));

bool HasKey<T,TKey>(Func<T,TKey>? k) => k is not null;

try { var grouped = source.GroupByUntil(k, d); }
catch (ArgumentNullException ex) when (ex.ParamName == "keySelector") { /* supply key selector */ }

When it happens

Trigger: Calling GroupByUntil with a null Func<TSource, TKey>, e.g. when the selector is supplied by a caller of a wrapper method that did not validate, or an expression tree converted to null.

Common situations: Public wrapper APIs forwarding an optional keySelector without checks; lambda built from reflection that failed; copied code where the selector line was deleted.

Related errors

• source
• Value cannot be null. (Parameter 'elementSelector')
• Value cannot be null. (Parameter 'source')
• action
• action (Parameter 'action')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/514e681f808e2f6b. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/GroupByUntil.cs:21

// See the LICENSE file in the project root for more information. 

using System.Collections.Concurrent;
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<IGroupedAsyncObservable<TKey, TSource>> GroupByUntil<TSource, TKey, TDuration>(this IAsyncObservable<TSource> source, Func<TSource, TKey> keySelector, Func<IGroupedAsyncObservable<TKey, TSource>, IAsyncObservable<TDuration>> durationSelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (durationSelector == null)
                throw new ArgumentNullException(nameof(durationSelector));

            return CreateAsyncObservable<IGroupedAsyncObservable<TKey, TSource>>.From(
                source,
                (keySelector, durationSelector),
                static (source, state, observer) => GroupByUntilCore<TSource, TKey, TSource, TDuration>(source, observer, (o, d) => AsyncObserver.GroupByUntil(o, d, state.keySelector, state.durationSelector)));
        }

        public static IAsyncObservable<IGroupedAsyncObservable<TKey, TSource>> GroupByUntil<TSource, TKey, TDuration>(this IAsyncObservable<TSource> source, Func<TSource, TKey> keySelector, Func<IGroupedAsyncObservable<TKey, TSource>, IAsyncObservable<TDuration>> durationSelector, IEqualityComparer<TKey> comparer)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (durationSelector == null)
                throw new ArgumentNullException(nameof(durationSelector));
            if (comparer == null)

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