dotnet/reactive · error · ArgumentNullException

until

Error message

until

What it means

AsyncObservable.SkipUntil<TSource, TUntil> throws ArgumentNullException when the until observable is null. The until sequence controls when the gate opens and starts forwarding source emissions; without it the operator cannot function, so it is validated before CreateAsyncObservable.From is invoked.

Solutions

1. Pass a valid IAsyncObservable<TUntil> as the until argument.
2. If the trigger is genuinely optional, use a never-completing placeholder observable instead of null.
3. Initialize the trigger stream before building the query.

Example fix

// before
var result = source.SkipUntil(triggerStream); // triggerStream null when no trigger registered
// after
var until = triggerStream ?? AsyncObservable.Never<int>();
var result = source.SkipUntil(until);

if (until is null) until = AsyncObservable.Never<TUntil>(); // or throw with context
var q = source.SkipUntil(until);

static bool HasTrigger<TUntil>(IAsyncObservable<TUntil>? u) => u is not null;

try { var q = source.SkipUntil(until); }
catch (ArgumentNullException ex) when (ex.ParamName == "until") { /* supply default trigger */ }

When it happens

Trigger: Calling source.SkipUntil(null), or passing a 'trigger' observable produced by a method that returned null (e.g. an event stream that is not yet wired up).

Common situations: Combining UI or signal streams where the trigger stream is optional and null in some flows; lazy initialization of the until observable that has not run yet; typos when wiring up multiple streams.

Related errors

• source
• ArgumentNullException
• observer
• scheduler
• Value cannot be null. (Parameter 'fourteenth')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/5e961b27de6d6a27. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/SkipUntil.cs:19

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Reactive.Concurrency;
using System.Reactive.Disposables;
using System.Threading;
using System.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<TSource> SkipUntil<TSource, TUntil>(this IAsyncObservable<TSource> source, IAsyncObservable<TUntil> until)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (until == null)
                throw new ArgumentNullException(nameof(until));

            return CreateAsyncObservable<TSource>.From(
                source,
                until,
                static async (source, until, observer) =>
                {
                    var (sourceObserver, untilObserver) = AsyncObserver.SkipUntil<TSource, TUntil>(observer);

                    var sourceTask = source.SubscribeSafeAsync(sourceObserver);
                    var untilTask = until.SubscribeSafeAsync(untilObserver);

                    // REVIEW: Consider concurrent subscriptions.

                    var d1 = await sourceTask.ConfigureAwait(false);
                    var d2 = await untilTask.ConfigureAwait(false);

                    return StableCompositeAsyncDisposable.Create(d1, d2);
                });

View on GitHub (pinned to 94b5d5ab91)