dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(observer))

Error message

ArgumentNullException(nameof(observer))

What it means

System.ArgumentNullException with ParamName 'observer' thrown by AsyncObserver.DistinctUntilChanged<TSource,TKey>(IAsyncObserver<TSource>, Func<TSource, ValueTask<TKey>>) when the observer is null. This overload accepts an async key selector returning ValueTask<TKey>; the observer is validated first and construction aborts on null.

Solutions

1. Pass the actual downstream observer instance.
2. Validate observer non-null before composing async-selector observers.
3. Fix the pipeline stage responsible for producing the observer.

Example fix

// before
var o = AsyncObserver.DistinctUntilChanged<Order, int>(null, async o => await GetKeyAsync(o));
// after
var o = AsyncObserver.DistinctUntilChanged<Order, int>(downstream, async o => await GetKeyAsync(o));

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

bool IsValidObserver<TSource>(IAsyncObserver<TSource>? o) => o is not null;

try
{
    var obs = AsyncObserver.DistinctUntilChanged<T, TKey>(observer, asyncKeySelector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "observer")
{
    // observer missing from pipeline — repair composition, not the args
    throw;
}

When it happens

Trigger: Calling AsyncObserver.DistinctUntilChanged<TSource,TKey>(null, asyncKeySelector) — the async-selector overload of the observer factory.

Common situations: Observer variable null because an earlier factory call returned null (should not happen with valid args) or a caller-supplied observer was never provided; test scaffolding passing null.

Related errors

• observer
• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'error')
• observer

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/042cb7b69e6b7012. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/DistinctUntilChanged.cs:123

                throw new ArgumentNullException(nameof(comparer));

            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);

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