dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'selector')

Error message

Value cannot be null. (Parameter 'selector')

What it means

The AsyncObserver.SelectMany overload (observer, Func<TSource, IAsyncObservable<TResult>> selector) throws ArgumentNullException because the selector parameter was null. The selector maps each source element to an inner async observable; a null selector leaves the operator unable to project elements, so it is rejected at construction time.

Solutions

1. Provide a real selector, e.g. x => AsyncObservable.Return(x * 2) or a method group like Project.
2. If the selector may be absent, fall back to an identity-based projection such as x => AsyncObservable.Empty<TResult>() rather than null.
3. Initialize delegate-typed members/fields with a default projection.
4. Validate at the call site before constructing the operator.

Example fix

// before
var (obs, disp) = AsyncObserver.SelectMany<TSource, TResult>(downstream, null);

// after
Func<TSource, IAsyncObservable<TResult>> selector = ProjectElement;
var (obs, disp) = AsyncObserver.SelectMany(downstream, selector);

if (observer is null) throw new ArgumentNullException(nameof(observer));
if (selector is null) throw new ArgumentNullException(nameof(selector));

static bool IsValid<TSource, TResult>(
    IAsyncObserver<TResult> observer,
    Func<TSource, IAsyncObservable<TResult>> selector)
    => observer is not null && selector is not null;

try
{
    var (obs, disp) = AsyncObserver.SelectMany(observer, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "selector")
{
    selector = x => AsyncObservable.Empty<TResult>();
}

When it happens

Trigger: Calling AsyncObserver.SelectMany(observer, null) with the sync-observable-selector overload, or passing a selector field/property that is still null (e.g. uninitialized delegate member).

Common situations: DI or configuration supplying the projection function lazily and failing, casting a method group from an object whose method is null, or refactoring where the selector argument was accidentally deleted.

Related errors

• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/2ff10f2f45ac91b9. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/SelectMany.cs:189

                static async (source, state, observer) =>
                {
                    var (sink, inner) = AsyncObserver.SelectMany(observer, state.collectionSelector, state.resultSelector);

                    var subscription = await source.SubscribeSafeAsync(sink).ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(subscription, inner);
                });
        }
    }

    public partial class AsyncObserver
    {
        public static (IAsyncObserver<TSource>, IAsyncDisposable) SelectMany<TSource, TResult>(IAsyncObserver<TResult> observer, Func<TSource, IAsyncObservable<TResult>> selector)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return SelectMany<TSource, TResult, TResult>(observer, x => new ValueTask<IAsyncObservable<TResult>>(selector(x)), (x, y) => new ValueTask<TResult>(y));
        }

        public static (IAsyncObserver<TSource>, IAsyncDisposable) SelectMany<TSource, TResult>(IAsyncObserver<TResult> observer, Func<TSource, ValueTask<IAsyncObservable<TResult>>> selector)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return SelectMany<TSource, TResult, TResult>(observer, selector, (x, y) => new ValueTask<TResult>(y));
        }

        public static (IAsyncObserver<TSource>, IAsyncDisposable) SelectMany<TSource, TCollection, TResult>(IAsyncObserver<TResult> observer, Func<TSource, IAsyncObservable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

View on GitHub (pinned to 94b5d5ab91)