dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'observer')

Error message

Value cannot be null. (Parameter 'observer')

What it means

This ArgumentNullException comes from AsyncObserver.FromAsyncPattern<TResult>(observer, begin, end) when the 'observer' parameter (IAsyncObserver<TResult> that receives the result or error) is null. This overload pushes the APM completion into an observer, so a null observer would make results undeliverable and is rejected immediately.

Solutions

1. Pass the actual IAsyncObserver<TResult> instance you received (e.g. the one handed to your Subscribe/Run method).
2. Verify the variable holding the observer is assigned before calling FromAsyncPattern.
3. If the observer is optional in your design, guard with a null check and fail with a descriptive error.

Example fix

// before
AsyncObserver.FromAsyncPattern<int>(null, cb => svc.BeginOp(cb, null), ar => svc.EndOp(ar));
// after
IAsyncObserver<int> observer = GetObserver(); // must be non-null
AsyncObserver.FromAsyncPattern(observer, cb => svc.BeginOp(cb, null), ar => svc.EndOp(ar));

if (observer is null) throw new ArgumentNullException(nameof(observer));
if (begin is null || end is null) throw new ArgumentException("begin/end required");
AsyncObserver.FromAsyncPattern(observer, begin, end);

static bool IsValidObserverPattern<TResult>(IAsyncObserver<TResult> observer,
    Func<AsyncCallback, object, IAsyncResult> begin, Func<IAsyncResult, TResult> end)
    => observer is not null && begin is not null && end is not null;

try { AsyncObserver.FromAsyncPattern(observer, begin, end); }
catch (ArgumentNullException ex) when (ex.ParamName == "observer")
{
    // observer pipeline produced null; inspect upstream Subscribe wiring
}

When it happens

Trigger: Calling AsyncObserver.FromAsyncPattern<TResult>(null, begin, end) — typically when wiring an observer from a pipeline that supplied null.

Common situations: Building custom async observables where the observer comes from SubscribeSafe/lambda plumbing that returned null; ordering mistakes when refactoring operator internals.

Related errors

• Value cannot be null. (Parameter 'onErrorAsync')
• Value cannot be null. (Parameter 'onCompletedAsync')
• Value cannot be null. (Parameter 'error')
• ArgumentNullException
• Value cannot be null. (Parameter 'onCompletedAsync')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/1eab5dcc0cf0ee4e. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromAsyncPattern.Generated.cs:740

                    AsyncObserver.FromAsyncPattern(subject, begin, end, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11, arg12, arg13, arg14);
                }
                catch (Exception ex)
                {
                    return Throw<Unit>(ex);
                }

                return subject.AsAsyncObservable();
            };
        }

    }

    public partial class AsyncObserver
    {
        public static IAsyncResult FromAsyncPattern<TResult>(IAsyncObserver<TResult> observer, Func<AsyncCallback, object, IAsyncResult> begin, Func<IAsyncResult, TResult> end)
        {
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));
            if (begin == null)
                throw new ArgumentNullException(nameof(begin));
            if (end == null)
                throw new ArgumentNullException(nameof(end));

            return begin(async iar =>
            {
                TResult result;

                try
                {
                    result = end(iar);
                }
                catch (Exception ex)
                {
                    await observer.OnErrorAsync(ex).ConfigureAwait(false);
                    return;
                }

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