{"record":{"id":"653a5414fe89b4cd","repo":"dotnet/reactive","slug":"nameof-addhandler","errorCode":null,"errorMessage":"nameof(addHandler)","messagePattern":"nameof\\(addHandler\\)","errorType":"validation","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs","lineNumber":69,"sourceCode":"        {\n            if (conversion == null)\n                throw new ArgumentNullException(nameof(conversion));\n            if (addHandler == null)\n                throw new ArgumentNullException(nameof(addHandler));\n            if (removeHandler == null)\n                throw new ArgumentNullException(nameof(removeHandler));\n            if (scheduler == null)\n                throw new ArgumentNullException(nameof(scheduler));\n\n            return FromEvent<TDelegate, object, TEventArgs>(action => conversion(new EventHandler<TEventArgs>((o, e) => action(o, e))), addHandler, removeHandler, scheduler).Select(t => new EventPattern<TEventArgs>(t.arg1, t.arg2));\n        }\n\n        public static IAsyncObservable<EventPattern<TSender, TEventArgs>> FromEventPattern<TDelegate, TSender, TEventArgs>(Action<TDelegate> addHandler, Action<TDelegate> removeHandler) => FromEventPattern<TDelegate, TSender, TEventArgs>(addHandler, removeHandler, GetSchedulerForCurrentContext());\n\n        public static IAsyncObservable<EventPattern<TSender, TEventArgs>> FromEventPattern<TDelegate, TSender, TEventArgs>(Action<TDelegate> addHandler, Action<TDelegate> removeHandler, IAsyncScheduler scheduler)\n        {\n            if (addHandler == null)\n                throw new ArgumentNullException(nameof(addHandler));\n            if (removeHandler == null)\n                throw new ArgumentNullException(nameof(removeHandler));\n            if (scheduler == null)\n                throw new ArgumentNullException(nameof(scheduler));\n\n            return FromEvent<TDelegate, TSender, TEventArgs>(action => ConvertDelegate<Action<TSender, TEventArgs>, TDelegate>(action), addHandler, removeHandler, scheduler).Select(t => new EventPattern<TSender, TEventArgs>(t.arg1, t.arg2));\n        }\n\n        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Action<EventHandler<TEventArgs>> addHandler, Action<EventHandler<TEventArgs>> removeHandler) => FromEventPattern<TEventArgs>(addHandler, removeHandler, GetSchedulerForCurrentContext());\n\n        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TEventArgs>(Action<EventHandler<TEventArgs>> addHandler, Action<EventHandler<TEventArgs>> removeHandler, IAsyncScheduler scheduler)\n        {\n            if (addHandler == null)\n                throw new ArgumentNullException(nameof(addHandler));\n            if (removeHandler == null)\n                throw new ArgumentNullException(nameof(removeHandler));\n            if (scheduler == null)\n                throw new ArgumentNullException(nameof(scheduler));","sourceCodeStart":51,"sourceCodeEnd":87,"githubUrl":"https://github.com/dotnet/reactive/blob/94b5d5ab912789f5abe9a72138a25bbd716fe59c/AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs#L51-L87","documentation":"System.ArgumentNullException with param name 'addHandler' (message records nameof(addHandler)). In the two-sender overload FromEventPattern<TDelegate, TSender, TEventArgs>(addHandler, removeHandler[, scheduler]) the first validation throws when addHandler is null; the FromEvent operator needs it to attach the converted handler that yields (sender, args) pairs.","triggerScenarios":"Calling FromEventPattern<TDelegate, TSender, TEventArgs>(null, removeHandler) or the scheduler variant with a null first Action<TDelegate>, e.g. misordered arguments or a reflection-built attach delegate that is null.","commonSituations":"Argument order mistakes (remove lambda passed first), reflection-based wiring failures producing a null delegate, or conditional wiring where the attach step is skipped for some events.","solutions":["Pass the attach delegate first: x => target.E += x.","Check argument order: addHandler precedes removeHandler in every overload.","Ensure dynamically created add delegates are non-null before calling.","Validate arguments at the call site with a clear assertion."],"exampleFix":"// before\nvar obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(\n    null,\n    x => target.E -= x);\n// after\nvar obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(\n    x => target.E += x,\n    x => target.E -= x);","handlingStrategy":"validation","validationCode":"if (addHandler == null) throw new ArgumentNullException(nameof(addHandler));","typeGuard":"static bool ValidSenderArgs<TDelegate>(Action<TDelegate> add, Action<TDelegate> remove) => add != null && remove != null;","tryCatchPattern":"try\n{\n    var obs = AsyncObservable.FromEventPattern<EventHandler, object, EventArgs>(add, remove);\n}\ncatch (ArgumentNullException ex) when (ex.ParamName == \"addHandler\")\n{\n    // first lambda was null; fix argument order or wiring\n}","preventionTips":["Pass the += lambda as the first argument.","Avoid swapping add/remove argument order during overloads.","Check reflection-built delegates for null before wiring.","Add tests that capture (sender, args) pairs to confirm the wiring."],"tags":["argument-null","reactive-extensions","events","dotnet"],"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"}