dotnet/reactive · error · ArgumentNullException

nameof(observer)

Error message

nameof(observer)

What it means

This is an ArgumentNullException thrown by AsyncObserver.ElementAtOrDefault when the observer argument is null. The factory guards its arguments so that a null observer fails fast at composition time rather than corrupting the subscription later. Null is never a valid observer here.

Solutions

1. Pass a valid IAsyncObserver<TSource> (from AsyncObserver.Create or an upstream operator)
2. Trace why the observer variable was null and fix that construction path
3. Assert non-null in dev builds before composing the pipeline

Example fix

// before
var op = AsyncObserver.ElementAtOrDefault(observer, 1); // observer null
// after
observer ??= AsyncObserver.Create<TSource>(...);
var op = AsyncObserver.ElementAtOrDefault(observer, 1);

if (observer is null) throw new ArgumentNullException(nameof(observer));
var op = AsyncObserver.ElementAtOrDefault(observer, index);

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

try { var op = AsyncObserver.ElementAtOrDefault(observer, index); }
catch (ArgumentNullException ex) when (ex.ParamName == "observer") { /* create/assign a real observer */ }

When it happens

Trigger: Calling AsyncObserver.ElementAtOrDefault<TSource>(null, index); typical when the observer is the result of a conditional chain or an uninitialized member at pipeline-build time.

Common situations: Hand-assembling async observer pipelines in tests; passing the result of a method that can return null; ordering mistakes where the observer is created after the operator call.

Related errors

• Value cannot be null. (Parameter 'observer')
• nameof(observer)
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'disposable1')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/6019b3cb694aa2a5. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/ElementAtOrDefault.cs:28

        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index));

            return Create(
                source,
                index,
                static (source, index, observer) => source.SubscribeSafeAsync(AsyncObserver.ElementAtOrDefault(observer, index)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> ElementAtOrDefault<TSource>(IAsyncObserver<TSource> observer, int index)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (index < 0)
                throw new ArgumentOutOfRangeException(nameof(index));

            return Create<TSource>(
                async x =>
                {
                    if (index-- == 0)
                    {
                        await observer.OnNextAsync(x).ConfigureAwait(false);
                        await observer.OnCompletedAsync().ConfigureAwait(false);
                    }
                },
                observer.OnErrorAsync,
                async () =>
                {
                    await observer.OnNextAsync(default).ConfigureAwait(false);
                    await observer.OnCompletedAsync().ConfigureAwait(false);
                }

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