{"record":{"id":"fb976ec37bc48055","repo":"dotnet/reactive","slug":"value-cannot-be-null-parameter-source-eventpatternsourcebase","errorCode":null,"errorMessage":"Value cannot be null. (Parameter 'source')","messagePattern":"Value cannot be null\\. \\(Parameter 'source'\\)","errorType":"validation","errorClass":"ArgumentNullException","httpStatus":null,"severity":"error","filePath":"Rx.NET/Source/src/System.Reactive/EventPatternSourceBase.cs","lineNumber":90,"sourceCode":"                        _isAdded = true;\n                    }\n                }\n            }\n        }\n\n        private readonly IObservable<EventPattern<TSender, TEventArgs>> _source;\n        private readonly Dictionary<Delegate, Stack<IDisposable>> _subscriptions = [];\n        private readonly Action<Action<TSender?, TEventArgs>, /*object,*/ EventPattern<TSender, TEventArgs>> _invokeHandler;\n\n        /// <summary>\n        /// Creates a new event pattern source.\n        /// </summary>\n        /// <param name=\"source\">Source sequence to expose as an event.</param>\n        /// <param name=\"invokeHandler\">Delegate used to invoke the event for each element of the sequence.</param>\n        /// <exception cref=\"ArgumentNullException\"><paramref name=\"source\"/> or <paramref name=\"invokeHandler\"/> is <c>null</c>.</exception>\n        protected EventPatternSourceBase(IObservable<EventPattern<TSender, TEventArgs>> source, Action<Action<TSender?, TEventArgs>, /*object,*/ EventPattern<TSender, TEventArgs>> invokeHandler)\n        {\n            _source = source ?? throw new ArgumentNullException(nameof(source));\n            _invokeHandler = invokeHandler ?? throw new ArgumentNullException(nameof(invokeHandler));\n        }\n\n        /// <summary>\n        /// Adds the specified event handler, causing a subscription to the underlying source.\n        /// </summary>\n        /// <param name=\"handler\">Event handler to add. The same delegate should be passed to the <see cref=\"Remove(Delegate)\"/> operation in order to remove the event handler.</param>\n        /// <param name=\"invoke\">Invocation delegate to raise the event in the derived class.</param>\n        /// <exception cref=\"ArgumentNullException\"><paramref name=\"handler\"/> or <paramref name=\"invoke\"/> is <c>null</c>.</exception>\n        protected void Add(Delegate handler, Action<TSender?, TEventArgs> invoke)\n        {\n            if (handler == null)\n            {\n                throw new ArgumentNullException(nameof(handler));\n            }\n\n            if (invoke == null)\n            {","sourceCodeStart":72,"sourceCodeEnd":108,"githubUrl":"https://github.com/dotnet/reactive/blob/94b5d5ab912789f5abe9a72138a25bbd716fe59c/Rx.NET/Source/src/System.Reactive/EventPatternSourceBase.cs#L72-L108","documentation":"The protected EventPatternSourceBase constructor requires a non-null source sequence. Passing null throws ArgumentNullException for 'source'. This base class backs FromEventPattern-style bridges that expose an observable as a .NET event, so a source is mandatory.","triggerScenarios":"A derived class (or user code subclassing EventPatternSourceBase) passing null for the IObservable<EventPattern<TSender,TEventArgs>> source parameter.","commonSituations":"Custom event-pattern adapters where the observable is produced by a factory that returned null; refactorings that dropped a null-coalescing operator on the source.","solutions":["Pass a valid non-null observable, e.g. Observable.FromEventPattern<...>(...) result.","Null-check or ?? Observable.Empty<EventPattern<TSender,TEventArgs>>() the value before calling the base constructor."],"exampleFix":"// before\nbase(source, invokeHandler)\n// after\nbase(source ?? Observable.Empty<EventPattern<TSender, TEventArgs>>(), invokeHandler)","handlingStrategy":"validation","validationCode":"if (source == null) throw new ArgumentNullException(nameof(source));","typeGuard":"bool IsValidSource<TSender, TArgs>(IObservable<EventPattern<TSender, TArgs>>? s) => s is not null;","tryCatchPattern":"try { new MyEventPatternSource(source, invoke); } catch (ArgumentNullException ex) when (ex.ParamName == \"source\") { /* supply fallback observable */ }","preventionTips":["Construct the observable before the event-pattern source, never lazily as a nullable field.","Coalesce with Observable.Empty<EventPattern<TSender,TEventArgs>>() for harmless defaults."],"tags":["argument-null","event-pattern","constructor"],"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"}