dotnet/reactive · error · ArgumentNullException

nameof(witness)

Error message

nameof(witness)

What it means

In the witness-based Do overload, the IObserver<TSource> witness receives copies of the pipeline's notifications and must be non-null. The library throws ArgumentNullException for witness (Do.cs:254) if it is null, because there would be no side-effect observer to invoke. Validation occurs before any subscription work.

Solutions

1. Pass a real witness observer; if diagnostics are optional, pass a no-op Observer stub instead of null.
2. Skip the Do call entirely when no witness is available (condition the pipeline construction).
3. Fix the factory/DI registration that was supposed to produce the witness observer.

Example fix

// before
var decorated = Do(src, diagnosticsObserver); // null when monitoring disabled
// after
var decorated = diagnosticsObserver != null ? Do(src, diagnosticsObserver) : src;

var decorated = witness != null ? Do(observer, witness) : observer; // skip witness when unavailable

static bool HasWitness<TSource>(IObserver<TSource>? w) => w is not null;

try
{
    var decorated = Do(observer, witness);
}
catch (ArgumentNullException ex) when (ex.ParamName == "witness")
{
    logger.LogWarning("No witness observer available; skipping Do side-effects");
    var decorated = observer;
}

When it happens

Trigger: Calling Do<TSource>(observer, witness) with a null witness, e.g. a side-effect logger or monitoring observer that was never constructed or that a factory returned as null.

Common situations: Optional diagnostics observer resolved from DI/config is null in environments without monitoring enabled; test scaffolding that passes null instead of a stub observer; conditional instrumentation code that ends up passing nothing.

Related errors

• nameof(onError)
• nameof(onCompleted)
• Value cannot be null. (Parameter 'observer')
• ArgumentNullException(nameof(observer))
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/9b0fe0c29c2dad2f. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Do.cs:254

        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return Do(observer, Create(onNext, onError, onCompleted));
        }

        public static IAsyncObserver<TSource> Do<TSource>(IAsyncObserver<TSource> observer, IObserver<TSource> witness)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (witness == null)
                throw new ArgumentNullException(nameof(witness));

            return Create<TSource>(
                async x =>
                {
                    try
                    {
                        witness.OnNext(x);
                    }
                    catch (Exception ex)
                    {
                        await observer.OnErrorAsync(ex).ConfigureAwait(false);
                        return;
                    }

                    await observer.OnNextAsync(x).ConfigureAwait(false);
                },
                async error =>
                {

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