dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(keySelector))

Error message

ArgumentNullException(nameof(keySelector))

What it means

ToLookup uses keySelector to group each element under a lookup key. A null keySelector makes grouping impossible, so ArgumentNullException(Parameter 'keySelector') is thrown synchronously at the call site before any subscription occurs.

Solutions

1. Supply a concrete key-selector function such as x => x.Key.
2. Validate any dynamically chosen selector for null before composing the query.
3. Centralize selector construction in a factory that throws a descriptive error instead of returning null.

Example fix

// before
var lookup = source.ToLookup(keySelector, x => x.Val); // keySelector null
// after
var lookup = source.ToLookup(x => x.Key, x => x.Val);

if (keySelector == null) throw new ArgumentException("keySelector must be provided", nameof(keySelector));

bool IsValidKeySelector<T,TKey>(Func<T,TKey> f) => f is not null;

try { var lookup = await source.ToLookup(keySelector, valueSelector); } catch (ArgumentNullException ex) when (ex.ParamName == "keySelector") { /* supply default selector */ }

When it happens

Trigger: Calling ToLookup(source, null, valueSelector) — e.g. a conditionally assigned key-selector delegate that ended up null.

Common situations: Dynamic query builders where the key selector is chosen per configuration and a missing mapping yields null; reflection-created delegates that failed silently.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• ArgumentNullException(nameof(keySelector))
• ArgumentNullException(nameof(observer))
• ArgumentNullException(nameof(source))
• ArgumentNullException(nameof(valueSelector))
• ArgumentNullException

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/75d36e0bbc5b31d6. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/ToLookup.cs:18

// 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.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<ILookup<TKey, TValue>> ToLookup<TSource, TKey, TValue>(this IAsyncObservable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> valueSelector)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (valueSelector == null)
                throw new ArgumentNullException(nameof(valueSelector));

            return CreateAsyncObservable<ILookup<TKey, TValue>>.From(
                source,
                (keySelector, valueSelector),
                static (source, state, observer) => source.SubscribeSafeAsync(AsyncObserver.ToLookup(observer, state.keySelector, state.valueSelector)));
        }

        public static IAsyncObservable<ILookup<TKey, TValue>> ToLookup<TSource, TKey, TValue>(this IAsyncObservable<TSource> source, Func<TSource, TKey> keySelector, Func<TSource, TValue> valueSelector, IEqualityComparer<TKey> comparer)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (valueSelector == null)
                throw new ArgumentNullException(nameof(valueSelector));
            if (comparer == null)

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