dotnet/reactive · error · ArgumentNullException

source

Error message

source

What it means

Distinct throws ArgumentNullException when the source IAsyncObservable<TSource> is null. The operator filters duplicate elements using a HashSet-backed observer and validates its source eagerly at composition time. Null source is a caller bug, not a recoverable condition.

Solutions

1. Ensure a non-null observable is passed to Distinct.
2. Fix the upstream producer that returned null instead of an observable.
3. Coalesce to AsyncObservable.Empty<TSource>() when null is an expected 'no data' case.

Example fix

// before
var distinct = AsyncObservable.Distinct(source); // source == null
// after
var distinct = source != null ? AsyncObservable.Distinct(source) : AsyncObservable.Empty<int>();

if (source is null) throw new InvalidOperationException("Distinct requires a non-null source");
var distinct = AsyncObservable.Distinct(source);

bool IsValidSource<T>(IAsyncObservable<T>? s) => s is not null;

try
{
    var distinct = AsyncObservable.Distinct(source);
}
catch (ArgumentNullException ex) when (ex.ParamName == "source")
{
    logger.LogError(ex, "Distinct called with null source");
    throw;
}

When it happens

Trigger: Calling AsyncObservable.Distinct(null) or invoking the extension method on a null reference (static form shown here takes source explicitly).

Common situations: Static invocation Distinct(source) where source comes from a nullable lookup; merging pipelines where one branch produced null.

Related errors

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

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/7e3a51ec6f507308. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Distinct.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.Collections.Generic;

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

            return Create(source, static (source, observer) => source.SubscribeSafeAsync(AsyncObserver.Distinct(observer)));
        }

        public static IAsyncObservable<TSource> Distinct<TSource>(IAsyncObservable<TSource> source, IEqualityComparer<TSource> comparer)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (comparer == null)
                throw new ArgumentNullException(nameof(comparer));

            return Create(
                source,
                comparer,
                static (source, comparer, observer) => source.SubscribeSafeAsync(AsyncObserver.Distinct(observer, comparer)));
        }
    }

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