What causes "Value cannot be null. (Parameter 'onNext')" in PrismLibrary/Prism?

Pass a valid Action callback to Subscribe, or use an empty lambda _ => { } if no action is needed.

PrismLibrary/Prism · error · ArgumentNullException

Value cannot be null. (Parameter 'onNext')

Error message

Value cannot be null. (Parameter 'onNext')

What it means

Prism's ObservableExtensions.Subscribe extension validates the onNext callback and throws ArgumentNullException(nameof(onNext)) when the Action<T> delegate passed is null. Subscribing without a handler action is not allowed.

Solutions

Pass a valid Action<T> callback to Subscribe, or use an empty lambda _ => { } if no action is needed.
Guard with onNext != null before calling Subscribe if the handler comes from external configuration.
Verify the delegate field/property holding the handler is assigned before subscription.

Example fix

// before
stream.Subscribe(_handler); // _handler is null
// after
stream.Subscribe(value => Handle(value));

Defensive patterns

Strategy: validation

Validate before calling

if (onNext is null) throw new InvalidOperationException("Subscribe requires an onNext handler");
source.Subscribe(onNext);

Try / catch

try { stream.Subscribe(handler); }
catch (ArgumentNullException ex) when (ex.ParamName == "onNext") { /* handler not configured */ }

Prevention

Never store handlers in fields that may be null at subscription time
Use a no-op lambda (_ => { }) when no action is required
Resolve handler delegates before wiring subscriptions

When it happens

Trigger: Passing a null Action<T> to source.Subscribe(onNext) — e.g. a method reference that is null, a conditionally assigned handler field, or reflection-built delegate that failed to resolve.

Common situations: Handler assigned from configuration or DI that wasn't set; passing a variable that's null on some code path; copy-pasting Subscribe overloads and accidentally dropping the lambda.

Related errors

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/81c8f211a07c5867. Report an issue: GitHub.

Appendix: source

Thrown at src/Avalonia/Prism.Avalonia/Extensions/ObservableExtensions.cs:26

    {
        /// <summary>
        /// Subscribes an element handler to an observable sequence.
        /// </summary>
        /// <typeparam name="T">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Observable sequence to subscribe to.</param>
        /// <param name="onNext">Action to invoke for each element in the observable sequence.</param>
        /// <returns><see cref="IDisposable"/> object used to unsubscribe from the observable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="onNext"/> is <c>null</c>.</exception>
        public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if (onNext == null)
            {
                throw new ArgumentNullException(nameof(onNext));
            }

            //
            // [OK] Use of unsafe Subscribe: non-pretentious constructor for an observer; this overload is not to be used internally.
            //
            return source.Subscribe/*Unsafe*/(new AnonymousObserver<T>(onNext, Stubs.Throw, Stubs.Nop));
        }
    }
}

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