dotnet/reactive · error · ArgumentNullException

ArgumentNullException(nameof(witness))

Error message

ArgumentNullException(nameof(witness))

What it means

The Do(observer, witness) overload throws ArgumentNullException when the witness argument is null. In this operator the witness is a second observer that receives a copy of every notification for side effects (logging, tracing); the library validates it up front because a null witness would crash the first time the source emits a notification. The primary observer and the witness are independently required.

Solutions

1. Pass a real witness observer; if no side effect is desired, drop the witness overload and use Do(observer) or pass AsyncObserver.Create<TSource>(_ => default, _ => default, () => default) as a no-op witness.
2. Verify that the factory/DI path producing the witness (e.g. a tracing observer built from a logger) cannot return null.
3. If the witness is optional in your design, guard with `witness ?? noopWitness` before composing.

Example fix

// before
var obs = AsyncObserver.Do<int>(downstream, traceObserver); // traceObserver null when tracing disabled

// after
var noopWitness = AsyncObserver.Create<int>(_ => default, _ => default, () => default);
var obs = AsyncObserver.Do<int>(downstream, traceObserver ?? noopWitness);

if (witness is null)
    witness = AsyncObserver.Create<T>(_ => default, _ => default, () => default); // no-op witness
var result = AsyncObserver.Do(source, observer, witness);

static bool IsUsableWitness<T>(IAsyncObserver<T>? w) => w is not null;

try
{
    var obs = AsyncObserver.Do(source, observer, witness);
}
catch (ArgumentNullException ex) when (ex.ParamName == "witness")
{
    log.LogWarning("tracing witness missing; falling back to plain Do");
    obs = AsyncObserver.Do(source, observer);
}

When it happens

Trigger: Calling AsyncObserver.Do<TSource>(validObserver, null) — a non-null observer but a null IAsyncObserver<TSource> witness.

Common situations: The witness (often a logging/tracing observer) is resolved lazily and is null because a logger was not configured; a conditional tracing observer is only built in some branches but Do is called unconditionally; a typo passes the wrong variable that is null.

Related errors

• nameof(observer)
• nameof(onNext)
• throw new ArgumentNullException(nameof(source));
• throw new ArgumentNullException(nameof(sampler));
• throw new ArgumentNullException(nameof(source));

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/fd67bbd0372052d2. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/Do.cs:157

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

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

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

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

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

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