dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(keySelector))

Error message

ArgumentNullException(nameof(keySelector))

What it means

System.ArgumentNullException with ParamName 'keySelector' thrown by AsyncObserver.DistinctUntilChanged<TSource,TKey>(IAsyncObserver<TSource>, Func<TSource, ValueTask<TKey>>) when the async key selector is null. The selector must produce a ValueTask<TKey> per element for consecutive-difference comparison.

Solutions

1. Provide a valid async selector: async x => (ValueTask<TKey>)ComputeKeyAsync(x).
2. Switch to the synchronous selector overload if no async work is needed.
3. Initialize the delegate before composing the observer.

Example fix

// before
Func<Event, ValueTask<string>> key = null;
var o = AsyncObserver.DistinctUntilChanged<Event, string>(observer, key);
// after
var o = AsyncObserver.DistinctUntilChanged<Event, string>(observer, async e => await ResolveKeyAsync(e));

if (asyncKeySelector is null) throw new ArgumentNullException(nameof(asyncKeySelector));
var obs = AsyncObserver.DistinctUntilChanged<T, TKey>(observer, asyncKeySelector);

bool IsValidAsyncSelector<TSource, TKey>(Func<TSource, ValueTask<TKey>>? f) => f is not null;

try
{
    var obs = AsyncObserver.DistinctUntilChanged<T, TKey>(observer, asyncKeySelector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "keySelector")
{
    var obs = AsyncObserver.DistinctUntilChanged<T, TKey>(observer, x => x.DefaultKey);
}

When it happens

Trigger: Calling AsyncObserver.DistinctUntilChanged<TSource,TKey>(observer, (Func<TSource, ValueTask<TKey>>)null).

Common situations: Async delegate resolved from config/registry was null; overload mismatch left the parameter bound to a null lambda variable; refactor dropped the selector assignment.

Related errors

• ArgumentNullException(nameof(observer))
• observer
• Argument cannot be null (Parameter name: observer)
• Argument cannot be null (Parameter name: predicate)
• Argument cannot be null (Parameter name: predicate)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/85a87ce85d429c0f. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/DistinctUntilChanged.cs:125

            return DistinctUntilChanged(observer, x => x, comparer);
        }

        public static IAsyncObserver<TSource> DistinctUntilChanged<TSource, TKey>(IAsyncObserver<TSource> observer, Func<TSource, TKey> keySelector)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));

            return DistinctUntilChanged(observer, keySelector, EqualityComparer<TKey>.Default);
        }

        public static IAsyncObserver<TSource> DistinctUntilChanged<TSource, TKey>(IAsyncObserver<TSource> observer, Func<TSource, ValueTask<TKey>> keySelector)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));

            return DistinctUntilChanged(observer, keySelector, EqualityComparer<TKey>.Default);
        }

        public static IAsyncObserver<TSource> DistinctUntilChanged<TSource, TKey>(IAsyncObserver<TSource> observer, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            var hasCurrentKey = false;
            var currentKey = default(TKey);

            return Create<TSource>(
                async x =>

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