dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source3')

Error message

Value cannot be null. (Parameter 'source3')

What it means

The Observable.Zip three-source overload throws ArgumentNullException when resultSelector is null. All source sequences were non-null; the combining function itself is the missing argument. Rx requires an explicit selector because it defines the output type and combination logic. Provide a non-null Func<TSource1,TSource2,TSource3,TResult>.

Solutions

1. Pass a valid lambda or method group as the result selector.
2. If the selector is resolved dynamically, default it (e.g. selector ?? ((a,b,c) => default(TResult))).
3. Check intermediate delegate fields/parameters for null before forwarding them to Zip.

Example fix

// before
Observable.Zip(a, b, c, mySelector); // mySelector is null
// after
var selector = mySelector ?? ((int x, int y, int z) => x + y + z);
Observable.Zip(a, b, c, selector);

if (resultSelector == null) throw new InvalidOperationException("resultSelector must be provided");

bool IsValid<T1,T2,T3,TResult>(Func<T1,T2,T3,TResult> f) => f is not null;

try { return Observable.Zip(a, b, c, resultSelector); }
catch (ArgumentNullException ex) when (ex.ParamName == "resultSelector") { /* supply default selector */ }

When it happens

Trigger: Calling Observable.Zip(source1, source2, source3, resultSelector) with a null selector delegate.

Common situations: Storing the selector in a variable resolved from config/DI that was never assigned; passing a method group from a class whose method is null via a delegate field; generic helper code forwarding a null selector parameter.

Related errors

• Value cannot be null. (Parameter 'source4')
• Value cannot be null. (Parameter 'source8')
• Value cannot be null. (Parameter 'source1')
• Value cannot be null. (Parameter 'source2')
• Value cannot be null. (Parameter 'resultSelector')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c4e36273931b17d1. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/Observable.Multiple.Zip.cs:38

        /// <param name="source3">Third observable source.</param>
        /// <param name="resultSelector">Function to invoke for each series of elements at corresponding indexes in the sources.</param>
        /// <returns>An observable sequence containing the result of combining elements of the sources using the specified result selector function.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source1"/> or <paramref name="source2"/> or <paramref name="source3"/> or <paramref name="resultSelector"/> is null.</exception>
        public static IObservable<TResult> Zip<TSource1, TSource2, TSource3, TResult>(this IObservable<TSource1> source1, IObservable<TSource2> source2, IObservable<TSource3> source3, Func<TSource1, TSource2, TSource3, TResult> resultSelector)
        {
            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 (resultSelector == null)
            {
                throw new ArgumentNullException(nameof(resultSelector));
            }

            return s_impl.Zip(source1, source2, source3, resultSelector);
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence by using the selector function whenever all of the observable sequences have produced an element at a corresponding index.
        /// </summary>
        /// <typeparam name="TSource1">The type of the elements in the first source sequence.</typeparam>
        /// <typeparam name="TSource2">The type of the elements in the second source sequence.</typeparam>
        /// <typeparam name="TSource3">The type of the elements in the third source sequence.</typeparam>
        /// <typeparam name="TSource4">The type of the elements in the fourth source sequence.</typeparam>
        /// <typeparam name="TResult">The type of the elements in the result sequence, returned by the selector function.</typeparam>

View on GitHub (pinned to 94b5d5ab91)