What causes "Value cannot be null. (Parameter 'predicate')" in dotnet/reactive?

Pass a concrete predicate lambda, e.g. x => x > 0.

dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'predicate')

Error message

Value cannot be null. (Parameter 'predicate')

What it means

The synchronous All operator throws ArgumentNullException when the predicate Func<TSource, bool> is null. The predicate decides whether the sequence 'matches all'; the library rejects a null predicate at construction so failures surface immediately with the parameter name.

Solutions

Pass a concrete predicate lambda, e.g. x => x > 0.
Supply a sensible default: predicate ?? (_ => true).
Guard optional predicates at the caller: if (predicate == null) throw new ArgumentNullException(nameof(predicate));

Example fix

// before
var ok = source.All(null);
// after
var ok = source.All(x => x > 0);

Defensive patterns

Strategy: validation

Validate before calling

if (predicate == null) throw new ArgumentNullException(nameof(predicate));
// or: predicate ??= (_ => true);

Type guard

static bool IsValidPredicate<T>(Func<T, bool> p) => p != null;

Try / catch

try { var q = source.All(predicate); }
catch (ArgumentNullException ex) when (ex.ParamName == "predicate") { /* use default predicate */ }

Prevention

Give optional predicate parameters non-null defaults (_ => true or _ => false).
Avoid conditional lambda construction that can yield null.
Pass lambdas directly at the call site instead of through nullable variables.

When it happens

Trigger: Calling source.All(null) or All(source, (Func<TSource, bool>)null), often when the predicate is stored in a variable, passed as an optional parameter, or built conditionally.

Common situations: Optional predicate parameters defaulted to null, predicate factories returning null on config misses, or refactors removing the lambda while keeping the call.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/4cab330cf207ce3e. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/All.cs:16

// 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.Threading.Tasks;

namespace System.Reactive.Linq
{
    public partial class AsyncObservable
    {
        public static IAsyncObservable<bool> All<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<bool>.From(
                source,
                predicate,
                static (source, predicate, observer) => source.SubscribeSafeAsync(AsyncObserver.All(observer, predicate)));
        }

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

View on GitHub (pinned to 94b5d5ab91)