dotnet/reactive · error · ArgumentNullException

observer

Error message

observer

What it means

System.ArgumentNullException with ParamName 'observer' thrown by AsyncObserver.DistinctUntilChanged<TSource>(IAsyncObserver<TSource>) when the downstream observer is null. This is the observer-side factory variant used when composing custom async operators. A null observer cannot receive OnNextAsync/OnErrorAsync, so construction fails immediately.

Solutions

1. Pass the actual non-null IAsyncObserver<TSource> from the enclosing subscription context.
2. Check why the observer variable is null — it should originate from SubscribeSafeAsync/Subscribe plumbing.
3. If writing an operator, propagate the observer argument rather than reconstructing it.

Example fix

// before
var obs = AsyncObserver.DistinctUntilChanged(downstreamObserver); // downstreamObserver was null
// after
if (downstreamObserver == null) throw new InvalidOperationException("observer not provided");
var obs = AsyncObserver.DistinctUntilChanged(downstreamObserver);

if (observer is null) throw new ArgumentNullException(nameof(observer));
var obs = AsyncObserver.DistinctUntilChanged(observer);

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

try
{
    var obs = AsyncObserver.DistinctUntilChanged(observer);
}
catch (ArgumentNullException ex) when (ex.ParamName == "observer")
{
    throw new InvalidOperationException("Downstream observer was not supplied", ex);
}

When it happens

Trigger: Calling AsyncObserver.DistinctUntilChanged(null) inside a custom operator's subscription chain.

Common situations: Wrapping an observer obtained from a caller that passed null; building operator chains where an earlier stage handed back a null observer; tests passing null to exercise the factory.

Related errors

• ArgumentNullException(nameof(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/c7a688118915a04c. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/DistinctUntilChanged.cs:95

                throw new ArgumentNullException(nameof(source));
            if (keySelector == null)
                throw new ArgumentNullException(nameof(keySelector));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            return Create(
                source,
                (keySelector, comparer),
                static (source, state, observer) => source.SubscribeSafeAsync(AsyncObserver.DistinctUntilChanged(observer, state.keySelector, state.comparer)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> DistinctUntilChanged<TSource>(IAsyncObserver<TSource> observer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return DistinctUntilChanged(observer, x => x, EqualityComparer<TSource>.Default);
        }

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

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