What causes "Argument cannot be null (Parameter name: observer)" in dotnet/reactive?

In custom operators, validate the observer with ArgumentNullException.ThrowIfNull before calling AsyncObserver.First

dotnet/reactive · error · ArgumentNullException

Argument cannot be null (Parameter name: observer)

Error message

Argument cannot be null (Parameter name: observer)

What it means

ArgumentNullException (Parameter name: observer) thrown by AsyncObserver.First<TSource>(IAsyncObserver<TSource>) when the downstream observer is null. This factory builds the observer-side logic that takes the first element then completes; it validates the observer before wiring callbacks. It surfaces through First/FirstAsync when internals or custom subscriptions pass a null observer.

Solutions

In custom operators, validate the observer with ArgumentNullException.ThrowIfNull before calling AsyncObserver.First
Ensure the caller (Create/SubscribeSafeAsync path) always supplies a non-null observer
When testing, pass a stub/test observer rather than null

Example fix

// before
public IAsyncObservable<T> MyOp(...) => ... AsyncObserver.First(observer); // observer may be null
// after
public IAsyncObservable<T> MyOp(...)
{
    ArgumentNullException.ThrowIfNull(observer);
    return ... AsyncObserver.First(observer);
}

Defensive patterns

Strategy: validation

Validate before calling

if (observer == null) throw new ArgumentNullException(nameof(observer));
var firstObserver = AsyncObserver.First(observer);

Type guard

bool HasObserver<T>(IAsyncObserver<T>? o) => o is not null;

Try / catch

try
{
    var firstObserver = AsyncObserver.First(observer);
}
catch (ArgumentNullException ex) when (ex.ParamName == nameof(observer))
{
    throw new InvalidOperationException("Downstream observer missing", ex);
}

Prevention

In custom operators, always validate the observer before composing
Never construct observers conditionally such that they can be null
Cover custom operator Subscribe paths with tests that pass a real observer

When it happens

Trigger: Calling AsyncObserver.First(null) directly, or implementing a custom IAsyncObservable/operator that passes a null observer into AsyncObserver.First — typically from a broken SubscribeSafeAsync path or a custom Create overload.

Common situations: Custom operator development where the observer argument is forwarded from a caller that supplied null; unit tests invoking AsyncObserver factories with null arguments.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/27c038a20d03f9b9. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/First.cs:55

        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return Create(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.First(observer, predicate)));
        }
    }

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

            return Create<TSource>(
                async x =>
                {
                    await observer.OnNextAsync(x).ConfigureAwait(false);
                    await observer.OnCompletedAsync().ConfigureAwait(false);
                },
                observer.OnErrorAsync,
                async () =>
                {
                    await observer.OnErrorAsync(new InvalidOperationException("The sequence is empty.")).ConfigureAwait(false);
                }
            );
        }

        public static IAsyncObserver<TSource> First<TSource>(IAsyncObserver<TSource> observer, Func<TSource, bool> predicate)
        {
            if (observer == null)

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