dotnet/reactive · error · ArgumentNullException

source

Error message

source

What it means

IsEmpty validates the source observable at operator-creation time; without a source there is nothing whose emptiness can be checked. Standard ArgumentNullException guard in the LINQ operator. Fix: pass a non-null IAsyncObservable<TSource>.

Solutions

1. Ensure the source IAsyncObservable<TSource> is non-null before calling IsEmpty; fall back to AsyncObservable.Empty<TSource>().
2. Trace the producer of the null observable and make it return an empty observable instead of null.
3. Add a null guard before the operator chain.

Example fix

// before
var empty = GetSource()?.IsEmpty(); // NullReference or ANE path
// after
var src = GetSource() ?? AsyncObservable.Empty<int>();
var empty = src.IsEmpty();

if (source is null) throw new ArgumentException("source must be non-null");
// or: source ??= AsyncObservable.Empty<TSource>();

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

try { var isEmpty = src.IsEmpty(); }
catch (ArgumentNullException ex) when (ex.ParamName == "source") { /* substitute Empty<TSource>() */ }

When it happens

Trigger: Calling source.IsEmpty() on a null receiver or AsyncObservable.IsEmpty(null), typically from a nullable observable obtained via lookup, config, or a null-returning method.

Common situations: Fluent chains built on results of methods that can return null (repository/cache lookups), especially when combining multiple query results.

Related errors

• source
• left
• null (Parameter 'scheduler')
• null (Parameter 'values')
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/abcbc52d94c4a493. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/IsEmpty.cs:12

// 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. 

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

            return Create<TSource, bool>(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.IsEmpty<TSource>(observer)));
        }
    }

    public partial class AsyncObserver
    {
        public static IAsyncObserver<TSource> IsEmpty<TSource>(IAsyncObserver<bool> observer)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return Any<TSource>(Select<bool, bool>(observer, b => !b));
        }
    }
}

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