dotnet/reactive · error · ArgumentNullException

comparer

Error message

comparer

What it means

The comparer-taking Distinct overload throws ArgumentNullException when the IEqualityComparer<TSource> argument is null. A comparer is required to decide element uniqueness; there is no null-comparer fallback in this overload (use the single-argument overload for default equality). The check runs before Create is invoked.

Solutions

1. Pass a concrete comparer such as EqualityComparer<TSource>.Default.
2. Use the Distinct(source) overload when you want default equality semantics.
3. Fix the comparer factory/registration so it never returns null.

Example fix

// before
source.Distinct(comparer); // comparer == null
// after
source.Distinct(comparer ?? EqualityComparer<int>.Default);

if (comparer is null) comparer = EqualityComparer<TSource>.Default;
var distinct = source.Distinct(comparer);

bool HasComparer<T>(IEqualityComparer<T>? c) => c is not null;

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

When it happens

Trigger: Calling source.Distinct(null) with a comparer-typed variable that was never assigned, or a comparer factory returning null.

Common situations: DI/config-driven comparer resolution returning null for an unknown key; conditional comparer building that skips assignment; tests passing null to use 'default' behavior incorrectly.

Related errors

• Value cannot be null. (Parameter 'comparer')
• source
• source
• ArgumentNullException(nameof(comparer))
• Value cannot be null. (Parameter 'comparer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/f4cb910968b7f3c1. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Distinct.cs:24

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)));
        }
    }

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

            return Distinct(observer, EqualityComparer<TSource>.Default);
        }

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