{"record":{"id":"aa0c8c3d6d3e83ba","repo":"dotnet/reactive","slug":"nameof-scheduler-fromeventpattern","errorCode":null,"errorMessage":"nameof(scheduler)","messagePattern":"nameof\\(scheduler\\)","errorType":"validation","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs","lineNumber":59,"sourceCode":"                throw new ArgumentNullException(nameof(removeHandler));\n            if (scheduler == null)\n                throw new ArgumentNullException(nameof(scheduler));\n\n            return FromEvent<TDelegate, object, TEventArgs>(action => ConvertDelegate<Action<object, TEventArgs>, TDelegate>(action), addHandler, removeHandler, scheduler).Select(t => new EventPattern<TEventArgs>(t.arg1, t.arg2));\n        }\n\n        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TDelegate, TEventArgs>(Func<EventHandler<TEventArgs>, TDelegate> conversion, Action<TDelegate> addHandler, Action<TDelegate> removeHandler) => FromEventPattern<TDelegate, TEventArgs>(conversion, addHandler, removeHandler, GetSchedulerForCurrentContext());\n\n        public static IAsyncObservable<EventPattern<TEventArgs>> FromEventPattern<TDelegate, TEventArgs>(Func<EventHandler<TEventArgs>, TDelegate> conversion, Action<TDelegate> addHandler, Action<TDelegate> removeHandler, IAsyncScheduler scheduler)\n        {\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","sourceCodeStart":41,"sourceCodeEnd":77,"githubUrl":"https://github.com/dotnet/reactive/blob/94b5d5ab912789f5abe9a72138a25bbd716fe59c/AsyncRx.NET/System.Reactive.Async/Linq/Operators/FromEventPattern.cs#L41-L77","documentation":"System.ArgumentNullException with param name 'scheduler' (message records nameof(scheduler)). The conversion-based overload with an explicit scheduler, FromEventPattern<TDelegate, TEventArgs>(conversion, addHandler, removeHandler, scheduler), ends its validation chain with a null check on scheduler and throws ArgumentNullException when it is null. Every other argument may be fine; only the scheduler is at fault.","triggerScenarios":"Calling the four-parameter conversion overload with null as the last IAsyncScheduler argument, or a scheduler variable resolved lazily from DI/config that is null at call time.","commonSituations":"Test setups where the scheduler field is not assigned, configuration that yields no scheduler, or an overload mix-up appending null where the parameterless-scheduler variant was intended.","solutions":["Pass a valid IAsyncScheduler (AsyncScheduler.Default, TestScheduler, DI-resolved instance).","Use the three-parameter conversion overload without scheduler to get the context scheduler automatically.","Fix scheduler initialization/registration so the referenced value is non-null.","Assert the scheduler before the call to fail at the right place."],"exampleFix":"// before\nvar obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(\n    h => h,\n    x => target.E += x,\n    x => target.E -= x,\n    null);\n// after\nvar obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(\n    h => h,\n    x => target.E += x,\n    x => target.E -= x,\n    AsyncScheduler.Default);","handlingStrategy":"validation","validationCode":"if (scheduler == null) throw new ArgumentNullException(nameof(scheduler)); // or omit to use context scheduler","typeGuard":"static bool ValidConversionArgs<TDelegate>(Func<EventHandler<TEventArgs>, TDelegate> conv, Action<TDelegate> add, Action<TDelegate> remove, IAsyncScheduler s)\n    => conv != null && add != null && remove != null && s != null;","tryCatchPattern":"try\n{\n    var obs = AsyncObservable.FromEventPattern<EventHandler<MyArgs>, MyArgs>(conversion, add, remove, scheduler);\n}\ncatch (ArgumentNullException ex) when (ex.ParamName == \"scheduler\")\n{\n    // scheduler null; default or fix initialization\n}","preventionTips":["Use the 3-argument conversion overload to avoid passing a scheduler.","Initialize scheduler fields before building pipelines.","Validate DI/config-provided schedulers at startup.","Never pass literal null as the final scheduler argument."],"tags":["argument-null","scheduler","reactive-extensions","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"}