dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'source')

Error message

Value cannot be null. (Parameter 'source')

What it means

The synchronous All operator (IAsyncObservable<bool> All<TSource>(source, Func<TSource, bool> predicate)) throws ArgumentNullException when the source observable is null. AsyncRx.NET validates operator arguments eagerly so that a null source fails at query construction instead of at subscription time.

Solutions

1. Ensure the source is created before the query: var src = AsyncObservable.FromArray(items); then src.All(x => ...).
2. Null-check or null-coalesce the source before composing: source ?? AsyncObservable.Empty<T>().
3. Fix the upstream producer so it never returns null (throw its own descriptive error instead).

Example fix

// before
IAsyncObservable<int> source = null;
var anyFail = source.All(x => x > 0); // throws
// after
var source = AsyncObservable.FromArray(new[] { 1, 2, 3 });
var anyFail = source.All(x => x > 0);

if (source == null) throw new ArgumentNullException(nameof(source));
// or: var safe = source ?? AsyncObservable.Empty<TSource>();

static bool HasSource<TSource>(IAsyncObservable<TSource> s) => s is not null;

try { var q = source.All(x => x > 0); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* initialize source and retry composition */ }

When it happens

Trigger: Calling source.All(predicate) via the extension method with source == null, or AsyncObservable.All(null, predicate) directly — e.g. a field holding the observable that was never assigned.

Common situations: Chained queries where an earlier factory returned null, lazy-initialized observables used before assignment, or null propagating from a configuration-provided source.

Related errors

• source
• Value cannot be null. (Parameter 'source')
• Value cannot be null. (Parameter 'disposable1')
• Value cannot be null. (Parameter 'disposable2')
• Value cannot be null. (Parameter 'onNextAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/7b88efe0cec71fe9. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/All.cs:14

// 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.Threading.Tasks;

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

            return CreateAsyncObservable<bool>.From(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.All(observer, predicate)));
        }

        public static IAsyncObservable<bool> All<TSource>(this IAsyncObservable<TSource> source, Func<TSource, ValueTask<bool>> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return CreateAsyncObservable<bool>.From(
                source,

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