{"record":{"id":"ca57ee34980aa704","repo":"dotnet/reactive","slug":"comparer-maxby","errorCode":null,"errorMessage":"comparer","messagePattern":"comparer","errorType":"exception","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"AsyncRx.NET/System.Reactive.Async/Linq/Operators/MaxBy.cs","lineNumber":32,"sourceCode":"            if (source == null)\n                throw new ArgumentNullException(nameof(source));\n            if (keySelector == null)\n                throw new ArgumentNullException(nameof(keySelector));\n\n            return CreateAsyncObservable<IList<TSource>>.From(\n                source,\n                keySelector,\n                static (source, keySelector, observer) => source.SubscribeSafeAsync(AsyncObserver.MaxBy(observer, keySelector)));\n        }\n\n        public static IAsyncObservable<IList<TSource>> MaxBy<TSource, TKey>(IAsyncObservable<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey> comparer)\n        {\n            if (source == null)\n                throw new ArgumentNullException(nameof(source));\n            if (keySelector == null)\n                throw new ArgumentNullException(nameof(keySelector));\n            if (comparer == null)\n                throw new ArgumentNullException(nameof(comparer));\n\n            return CreateAsyncObservable<IList<TSource>>.From(\n                source,\n                (keySelector, comparer),\n                static (source, state, observer) => source.SubscribeSafeAsync(AsyncObserver.MaxBy(observer, state.keySelector, state.comparer)));\n        }\n\n        public static IAsyncObservable<IList<TSource>> MaxBy<TSource, TKey>(IAsyncObservable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector)\n        {\n            if (source == null)\n                throw new ArgumentNullException(nameof(source));\n            if (keySelector == null)\n                throw new ArgumentNullException(nameof(keySelector));\n\n            return CreateAsyncObservable<IList<TSource>>.From(\n                source,\n                keySelector,\n                static (source, keySelector, observer) => source.SubscribeSafeAsync(AsyncObserver.MaxBy(observer, keySelector)));","sourceCodeStart":14,"sourceCodeEnd":50,"githubUrl":"https://github.com/dotnet/reactive/blob/94b5d5ab912789f5abe9a72138a25bbd716fe59c/AsyncRx.NET/System.Reactive.Async/Linq/Operators/MaxBy.cs#L14-L50","documentation":"The three-argument sync MaxBy overload requires an IComparer<TKey> to order keys. A null comparer is rejected with ArgumentNullException before subscription so the failure happens at call time, not during enumeration.","triggerScenarios":"Calling AsyncObservable.MaxBy(source, keySelector, comparer) with comparer == null (e.g. an unset comparer field or a GetComparer() that returned null).","commonSituations":"DI/config supplies an optional comparer that is absent; a dictionary lookup for a comparer returns null; misuse of the overload instead of the comparer-less one.","solutions":["Pass Comparer<TKey>.Default or Comparer<TKey>.Create(...) explicitly","Use the MaxBy(source, keySelector) overload which defaults to Comparer<TKey>.Default","Ensure any comparer factory/registry returns a non-null instance"],"exampleFix":"// before\nawait source.MaxBy(x => x.Score, null);\n// after\nawait source.MaxBy(x => x.Score, Comparer<int>.Default);","handlingStrategy":"validation","validationCode":"var cmp = comparer ?? Comparer<TKey>.Default;\n// then call: source.MaxBy(keySelector, cmp)","typeGuard":"bool HasComparer<TKey>(IComparer<TKey> cmp) => cmp is not null;","tryCatchPattern":"try\n{\n    result = source.MaxBy(keySelector, comparer);\n}\ncatch (ArgumentNullException ex) when (ex.ParamName == \"comparer\")\n{\n    result = source.MaxBy(keySelector); // fall back to default comparer\n}","preventionTips":["Default to Comparer<TKey>.Default explicitly when no custom ordering is needed","Never pass null as an IComparer argument; use overloads designed for no comparer","Validate comparer factories/registries return non-null instances"],"tags":["argument-null","comparer","async-rx"],"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"}