dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'selector')

Error message

Value cannot be null. (Parameter 'selector')

What it means

The 7-source Zip overload with a result selector Func<T1..T7, ValueTask<TResult>> throws ArgumentNullException('selector') when the selector delegate is null. After checking all seven sources, the operator requires a non-null function to combine the zipped elements (it is invoked by AsyncObserver.Zip to project each tuple).

Solutions

1. Supply a valid async selector, e.g. (t1, t2, t3, t4, t5, t6, t7) => ValueTask.FromResult(t1 + t7).
2. If no projection is needed, use the plain 7-source Zip overload that returns a tuple instead of passing null.
3. Guard the selector variable for null before calling Zip.

Example fix

// before
Func<T1,T2,T3,T4,T5,T6,T7,ValueTask<TResult>> selector = maybeSelector;
var zipped = AsyncObservable.Zip(s1, s2, s3, s4, s5, s6, s7, selector);
// after
var zipped = AsyncObservable.Zip(s1, s2, s3, s4, s5, s6, s7,
    maybeSelector ?? ((t1, t2, t3, t4, t5, t6, t7) => ValueTask.FromResult(project(t1, t2, t3, t4, t5, t6, t7))));

if (selector == null) throw new InvalidOperationException("zip result selector must be provided");

bool IsValidSelector<T1,T2,T3,T4,T5,T6,T7,TResult>(Func<T1,T2,T3,T4,T5,T6,T7,ValueTask<TResult>> f) => f is not null;

try
{
    var zipped = AsyncObservable.Zip(s1, s2, s3, s4, s5, s6, s7, selector);
}
catch (ArgumentNullException ex) when (ex.ParamName == "selector")
{
    logger.LogError(ex, "zip selector was null");
}

When it happens

Trigger: Calling AsyncObservable.Zip(s1..s7, (Func<T1,...,T7,ValueTask<TResult>>)null) - passing null where the async result-selector lambda should be.

Common situations: A selector stored in a variable or returned from a factory was null; conditional lambda assignment skipped; refactoring from the no-selector overload to the selector overload left the argument empty.

Related errors

• throw new ArgumentNullException(nameof(selector));
• Value cannot be null. (Parameter 'selector')
• Value cannot be null. (Parameter 'keySelector')
• Value cannot be null. (Parameter 'source4')
• Value cannot be null. (Parameter 'source5')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/db55dc58b6e1d8b8. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Zip.Generated.cs:508

        public static IAsyncObservable<TResult> Zip<T1, T2, T3, T4, T5, T6, T7, TResult>(this IAsyncObservable<T1> source1, IAsyncObservable<T2> source2, IAsyncObservable<T3> source3, IAsyncObservable<T4> source4, IAsyncObservable<T5> source5, IAsyncObservable<T6> source6, IAsyncObservable<T7> source7, Func<T1, T2, T3, T4, T5, T6, T7, TResult> selector)
        {
            if (source1 == null)
                throw new ArgumentNullException(nameof(source1));
            if (source2 == null)
                throw new ArgumentNullException(nameof(source2));
            if (source3 == null)
                throw new ArgumentNullException(nameof(source3));
            if (source4 == null)
                throw new ArgumentNullException(nameof(source4));
            if (source5 == null)
                throw new ArgumentNullException(nameof(source5));
            if (source6 == null)
                throw new ArgumentNullException(nameof(source6));
            if (source7 == null)
                throw new ArgumentNullException(nameof(source7));
            if (selector == null)
                throw new ArgumentNullException(nameof(selector));

            return Create<TResult>(async observer =>
            {
                var d = new CompositeAsyncDisposable();

                var (observer1, observer2, observer3, observer4, observer5, observer6, observer7) = AsyncObserver.Zip(observer, selector);

                var sub1 = source1.SubscribeSafeAsync(observer1).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub2 = source2.SubscribeSafeAsync(observer2).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub3 = source3.SubscribeSafeAsync(observer3).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub4 = source4.SubscribeSafeAsync(observer4).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub5 = source5.SubscribeSafeAsync(observer5).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub6 = source6.SubscribeSafeAsync(observer6).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();
                var sub7 = source7.SubscribeSafeAsync(observer7).AsTask().ContinueWith(disposable => d.AddAsync(disposable.Result).AsTask()).Unwrap();

                await Task.WhenAll(sub1, sub2, sub3, sub4, sub5, sub6, sub7).ConfigureAwait(false);

                return d;

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