dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'predicate')

Error message

Value cannot be null. (Parameter 'predicate')

What it means

The predicate overloads of LongCount require a non-null filter delegate; the sync overload at LongCount.cs:24 throws ArgumentNullException when Func<TSource, bool> predicate is null. Without a predicate the operator cannot decide which elements to count, so it fails fast.

Solutions

1. Pass a concrete predicate, e.g. x => x.IsActive.
2. If no filtering is desired, call the parameterless source.LongCount() overload instead of passing null.
3. Default null delegates to a pass-through filter: predicate ?? (_ => true).

Example fix

// before
source.LongCount(predicate); // predicate is null
// after
source.LongCount(predicate ?? (_ => true));

if (predicate is null) predicate = static _ => true;
var count = source.LongCount(predicate);

static bool HasPredicate<T>(Func<T, bool>? p) => p is not null;

try { var count = source.LongCount(predicate); }
catch (ArgumentNullException ex) when (ex.ParamName == "predicate") { var count = source.LongCount(); }

When it happens

Trigger: Calling source.LongCount((Func<T, bool>)null) — the sync-predicate overload at LongCount.cs:24 with a null predicate delegate.

Common situations: Storing a filter in a field/variable that was never initialized; passing the result of a conditional expression like cond ? myFilter : null; reflection-driven code resolving a delegate that came back null.

Related errors

• conversion
• addHandler
• Value cannot be null. (Parameter 'condition')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'source')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/6aaed6d82fb47dd3. Report an issue: GitHub.

Appendix: source

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

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

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

        public static IAsyncObservable<long> LongCount<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<long>.From(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.LongCount(observer, predicate)));
        }

        public static IAsyncObservable<long> LongCount<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<long>.From(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.LongCount(observer, predicate)));

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