dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(onError))

Error message

ArgumentNullException(nameof(onError))

What it means

The Do(source, onError) operator throws ArgumentNullException because the onError Action<Exception> side-effect callback is null. The library requires a real callback since Do exists only to invoke it.

Solutions

1. Pass a non-null Action<Exception>, even a no-op such as _ => { }
2. Guard the handler with a default before composing: handler ?? DefaultHandler
3. Fix the producer that was supposed to supply the error handler

Example fix

// before
Action<Exception> onError = config.ErrorHandler; // may be null
var res = src.Do(onError);
// after
var res = src.Do(config.ErrorHandler ?? (ex => { }));

if (onError is null) onError = _ => { };
var res = source.Do(onError);

static bool IsValidHandler(Action<Exception> h) => h is not null;

try { var res = source.Do(onError); }
catch (ArgumentNullException ex) when (ex.ParamName == "onError") { /* fall back to no-op handler */ }

When it happens

Trigger: Calling source.Do(onError) with a null delegate, e.g. a conditional handler variable that was never assigned or a method parameter forwarded as null.

Common situations: Optional logging/error-handler fields that default to null; DI-injected handlers missing; passing a local that failed to initialize.

Related errors

• ArgumentNullException(nameof(onCompleted))
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'onNext')
• Value cannot be null. (Parameter 'onError')
• nameof(finallyAction)

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/49c7155eda900bff. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Do.cs:42

        public static IAsyncObservable<TSource> Do<TSource>(this IAsyncObservable<TSource> source, Action<TSource> onNext)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));

            return Create(
                source,
                onNext,
                static (source, onNext, target) => source.SubscribeSafeAsync(AsyncObserver.Do(target, onNext)));
        }

        public static IAsyncObservable<TSource> Do<TSource>(this IAsyncObservable<TSource> source, Action<Exception> onError)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));

            return Create(
                source,
                onError,
                static (source, onError, target) => source.SubscribeSafeAsync(AsyncObserver.Do(target, onError)));
        }

        public static IAsyncObservable<TSource> Do<TSource>(this IAsyncObservable<TSource> source, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return Create(
                source,
                onCompleted,
                static (source, onCompleted, target) => source.SubscribeSafeAsync(AsyncObserver.Do(target, onCompleted)));

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