{"record":{"id":"c0e05e9252f4c875","repo":"dotnet/reactive","slug":"value-cannot-be-null-parameter-begin","errorCode":null,"errorMessage":"Value cannot be null. (Parameter 'begin')","messagePattern":"Value cannot be null\\. \\(Parameter 'begin'\\)","errorType":"exception","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromAsyncPattern.Generated.cs","lineNumber":520,"sourceCode":"                var subject = new SequentialAsyncAsyncSubject<Unit>();\n\n                try\n                {\n                    AsyncObserver.FromAsyncPattern(subject, begin, end, arg1, arg2, arg3, arg4, arg5);\n                }\n                catch (Exception ex)\n                {\n                    return Throw<Unit>(ex);\n                }\n\n                return subject.AsAsyncObservable();\n            };\n        }\n\n        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)\n        {\n            if (begin == null)\n                throw new ArgumentNullException(nameof(begin));\n            if (end == null)\n                throw new ArgumentNullException(nameof(end));\n\n            return (arg1, arg2, arg3, arg4, arg5, arg6) =>\n            {\n                var subject = new SequentialAsyncAsyncSubject<Unit>();\n\n                try\n                {\n                    AsyncObserver.FromAsyncPattern(subject, begin, end, arg1, arg2, arg3, arg4, arg5, arg6);\n                }\n                catch (Exception ex)\n                {\n                    return Throw<Unit>(ex);\n                }\n\n                return subject.AsAsyncObservable();\n            };","sourceCodeStart":502,"sourceCodeEnd":538,"githubUrl":"https://github.com/dotnet/reactive/blob/94b5d5ab912789f5abe9a72138a25bbd716fe59c/AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromAsyncPattern.Generated.cs#L502-L538","documentation":"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.","triggerScenarios":"Calling FromAsyncPattern(begin, end) with begin == null, e.g. passing a reflected Begin method delegate that was never bound or a variable left uninitialized.","commonSituations":"Reflection-based binding of legacy BeginXxx methods failing silently ( MethodInfo.CreateDelegate returned null); codegen/config-driven wiring where the begin entry is missing.","solutions":["Pass a properly bound begin delegate, e.g. svc.BeginOp as a method group.","Check reflection results: if methodInfo == null or the created delegate is null, fail earlier with a clear message.","Ensure the target service/object that owns the Begin method is initialized before constructing the pattern."],"exampleFix":"// before\nvar begin = (Func<...>)null;\nvar f = AsyncObservable.FromAsyncPattern<...>(begin, end);\n// after\nvar f = AsyncObservable.FromAsyncPattern<...>(svc.BeginOp, svc.EndOp);","handlingStrategy":"validation","validationCode":"if (begin is null) throw new InvalidOperationException(\"begin delegate missing; check reflection binding\");","typeGuard":"bool HasBegin(Delegate begin) => begin is not null;","tryCatchPattern":"try { var f = AsyncObservable.FromAsyncPattern<T1,T2,T3,T4,T5,T6>(begin, end); }\ncatch (ArgumentNullException ex) when (ex.ParamName == \"begin\") { /* fix binding, then retry construction */ }","preventionTips":["Fail fast when MethodInfo-based delegate creation yields null","Keep Begin method owner objects initialized before pattern construction","Prefer method groups over runtime-built delegates for APM adapters"],"tags":["argument-null","apm","fromasyncpattern","generated-code"],"backgroundTag":"null-argument","analyzedSha":"94b5d5ab912789f5abe9a72138a25bbd716fe59c","analyzedAt":"2026-09-15T02:26:24.759Z","contentChangedAt":"2026-09-15T02:26:24.759Z","schemaVersion":2},"datasetVersion":"2026-09-15T23:17:13.987Z"}