dotnet/reactive · error · ArgumentNullException

first

Error message

first

What it means

QbservableEx.CombineLatest<TFirst,TSecond>(IQbservable<TFirst>, IObservable<TSecond>) throws ArgumentNullException when the first (qbservable) source is null. The method uses first.Provider and first.Expression to build the resulting query, so a null first sequence is rejected before expression construction.

Solutions

1. Ensure the primary qbservable source is produced by a valid IQbservableProvider before combining.
2. Null-check first (and second) before calling CombineLatest.
3. If the first source can be absent, substitute an empty/never qbservable or restructure so a non-null source always exists.

Example fix

// before
IQbservable<int> first = null;
var combined = first.CombineLatest(second);
// after
var first = provider.FromNumbers();
var combined = first.CombineLatest(second);

if (first is null || second is null) throw new InvalidOperationException("CombineLatest requires non-null first and second sources.");

static bool IsCombinable<T>(IQbservable<T> q) => q is not null && q.Provider is not null;

try { var combined = first.CombineLatest(second); }
catch (ArgumentNullException ex) when (ex.ParamName == "first") { /* rebuild the primary qbservable */ }

When it happens

Trigger: Calling first.CombineLatest(second) where first is a null IQbservable<TFirst>; e.g. a pipeline stage that produced null instead of a qbservable, or passing null as the first argument of the static call.

Common situations: Conditional query construction where the first branch was skipped; mocks in tests returning null; chaining off results of provider.CreateQuery paths that failed silently upstream.

Related errors

• source
• keySelector
• Value cannot be null. (Parameter 'keySelector')
• Value cannot be null. (Parameter 'comparer')
• Value cannot be null. (Parameter 'first')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/6a8155da07ade2d3. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/Linq/QbservableEx.Homoicon.cs:35

namespace System.Reactive.Linq
{
    public static partial class QbservableEx
    {
        /// <summary>
        /// Merges the specified observable sequences into one observable sequence of tuple values whenever any of the observable sequences produces an element.
        /// </summary>
        /// <typeparam name="TFirst">The type of the elements in the first source sequence.</typeparam>
        /// <typeparam name="TSecond">The type of the elements in the second source sequence.</typeparam>
        /// <param name="first">First observable source.</param>
        /// <param name="second">Second observable source.</param>
        /// <returns>An observable sequence containing the result of combining elements of the sources using tuple values.</returns>
        /// <exception cref="ArgumentNullException">
        /// <paramref name="first" /> or <paramref name="second" /> is null.</exception>
        public static IQbservable<(TFirst First, TSecond Second)> CombineLatest<TFirst, TSecond>(this IQbservable<TFirst> first, IObservable<TSecond> second)
        {
            if (first == null)
                throw new ArgumentNullException(nameof(first));
            if (second == null)
                throw new ArgumentNullException(nameof(second));

            return first.Provider.CreateQuery<(TFirst First, TSecond Second)>(
                Expression.Call(
                    null,
                    new Func<IQbservable<TFirst>, IObservable<TSecond>, IQbservable<(TFirst First, TSecond Second)>>(CombineLatest<TFirst, TSecond>).Method,
                    first.Expression,
                    GetSourceExpression(second)
                )
            );
        }

        /// <summary>
        /// Merges the specified observable sequences into one observable sequence of tuple values whenever any of the observable sequences produces an element.
        /// </summary>
        /// <typeparam name="TFirst">The type of the elements in the first source sequence.</typeparam>
        /// <typeparam name="TSecond">The type of the elements in the second source sequence.</typeparam>

View on GitHub (pinned to 94b5d5ab91)