dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'comparer')

Error message

Value cannot be null. (Parameter 'comparer')

What it means

The comparer overload of AsyncObservable.ToHashSet requires a non-null IEqualityComparer<TSource> used to build the internal HashSet and to deduplicate incoming elements. Passing null throws ArgumentNullException(Parameter 'comparer') synchronously at the call site.

Solutions

1. Call the parameterless ToHashSet(source) overload when default equality is desired.
2. Pass EqualityComparer<TSource>.Default explicitly instead of null.
3. Fix the comparer construction so it returns a real instance rather than null.

Example fix

// before
var res = source.ToHashSet(comparer); // comparer is null
// after
var res = comparer != null ? source.ToHashSet(comparer) : source.ToHashSet();

if (comparer == null) comparer = EqualityComparer<TSource>.Default;

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

try { var set = await source.ToHashSet(comparer); } catch (ArgumentNullException ex) when (ex.ParamName == "comparer") { /* retry with default comparer */ }

When it happens

Trigger: Calling ToHashSet(source, comparer) with comparer = null, typically a comparer variable that failed to initialize or a ternary that fell through to null.

Common situations: Injecting a comparer via DI that was not registered; loading a comparer from configuration; passing null intending to mean 'use default comparer' instead of calling the parameterless overload.

Related errors

• Value cannot be null. (Parameter 'comparer')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onError')
• source

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/1d174722d12db036. Report an issue: GitHub.

Appendix: source

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

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

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

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

            return CreateAsyncObservable<HashSet<TSource>>.From(
                source,
                comparer,
                static (source, comparer, observer) => source.SubscribeSafeAsync(AsyncObserver.ToHashSet(observer, comparer)));
        }
    }

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

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

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