dotnet/reactive · error · ArgumentNullException

Value cannot be null. (Parameter 'begin')

Error message

Value cannot be null. (Parameter 'begin')

What it means

FromAsyncPattern<T1..T6> validates that the begin delegate (Func<..., AsyncCallback, object, IAsyncResult>) is non-null and throws ArgumentNullException with parameter name 'begin' otherwise. The begin delegate initiates the underlying APM operation, so a null makes the returned function unusable and is rejected at pattern-construction time. This is the six-generic-argument generated overload.

Solutions

1. Pass a properly bound begin delegate, e.g. svc.BeginOp as a method group.
2. Check reflection results: if methodInfo == null or the created delegate is null, fail earlier with a clear message.
3. Ensure the target service/object that owns the Begin method is initialized before constructing the pattern.

Example fix

// before
var begin = (Func<...>)null;
var f = AsyncObservable.FromAsyncPattern<...>(begin, end);
// after
var f = AsyncObservable.FromAsyncPattern<...>(svc.BeginOp, svc.EndOp);

if (begin is null) throw new InvalidOperationException("begin delegate missing; check reflection binding");

bool HasBegin(Delegate begin) => begin is not null;

try { var f = AsyncObservable.FromAsyncPattern<T1,T2,T3,T4,T5,T6>(begin, end); }
catch (ArgumentNullException ex) when (ex.ParamName == "begin") { /* fix binding, then retry construction */ }

When it happens

Trigger: Calling FromAsyncPattern(begin, end) with begin == null, e.g. passing a reflected Begin method delegate that was never bound or a variable left uninitialized.

Common situations: Reflection-based binding of legacy BeginXxx methods failing silently ( MethodInfo.CreateDelegate returned null); codegen/config-driven wiring where the begin entry is missing.

Related errors

• Value cannot be null. (Parameter 'end')
• Value cannot be null. (Parameter 'begin')
• Value cannot be null. (Parameter 'subscribeAsync')
• Value cannot be null. (Parameter 'observer')
• Value cannot be null. (Parameter 'observer')

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/c0e05e9252f4c875. Report an issue: GitHub.

Appendix: source

Thrown at AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromAsyncPattern.Generated.cs:520

                var subject = new SequentialAsyncAsyncSubject<Unit>();

                try
                {
                    AsyncObserver.FromAsyncPattern(subject, begin, end, arg1, arg2, arg3, arg4, arg5);
                }
                catch (Exception ex)
                {
                    return Throw<Unit>(ex);
                }

                return subject.AsAsyncObservable();
            };
        }

        public static Func<T1, T2, T3, T4, T5, T6, IAsyncObservable<Unit>> FromAsyncPattern<T1, T2, T3, T4, T5, T6>(Func<T1, T2, T3, T4, T5, T6, AsyncCallback, object, IAsyncResult> begin, Func<IAsyncResult, Unit> end)
        {
            if (begin == null)
                throw new ArgumentNullException(nameof(begin));
            if (end == null)
                throw new ArgumentNullException(nameof(end));

            return (arg1, arg2, arg3, arg4, arg5, arg6) =>
            {
                var subject = new SequentialAsyncAsyncSubject<Unit>();

                try
                {
                    AsyncObserver.FromAsyncPattern(subject, begin, end, arg1, arg2, arg3, arg4, arg5, arg6);
                }
                catch (Exception ex)
                {
                    return Throw<Unit>(ex);
                }

                return subject.AsAsyncObservable();
            };

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