MA0093: do not offer EventArgs.Empty when the event args are generic

src/Meziantou.Analyzer.CodeFixers/Rules/UseEventArgsEmptyFixer.cs

@@ -13,28 +13,29 @@ public override FixAllProvider GetFixAllProvider()
     public override async Task RegisterCodeFixesAsync(CodeFixContext context)
     {
         var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);
-        var nodeToFix = root?.FindNode(context.Span, getInnermostNodeForTie: true);
-        if (nodeToFix is null)
+        if (root?.FindNode(context.Span, getInnermostNodeForTie: true) is not ExpressionSyntax nodeToFix)
+            return;
+
+        var semanticModel = await context.Document.GetSemanticModelAsync(context.CancellationToken).ConfigureAwait(false);
+        if (semanticModel?.Compilation.GetTypeByMetadataName("System.EventArgs") is not { } eventArgsSymbol)
+            return;
+
+        // The parameter can be typed with a type parameter constrained to EventArgs: Invoke(this, null) cannot become Invoke(this, EventArgs.Empty)
+        if (semanticModel.GetTypeInfo(nodeToFix, context.CancellationToken).ConvertedType is { } convertedType && !semanticModel.Compilation.ClassifyConversion(eventArgsSymbol, convertedType).IsImplicit)
             return;
 
         context.RegisterCodeFix(
             CodeAction.Create(
                 "Use EventArgs.Empty",
-                ct => Fix(context, nodeToFix, ct),
+                ct => Fix(context.Document, nodeToFix, eventArgsSymbol, ct),
                 equivalenceKey: "Use EventArgs.Empty"),
             context.Diagnostics);
     }
 
-    private static async Task<Document> Fix(CodeFixContext context, SyntaxNode nodeToFix, CancellationToken cancellationToken)
+    private static async Task<Document> Fix(Document document, ExpressionSyntax nodeToFix, INamedTypeSymbol eventArgsSymbol, CancellationToken cancellationToken)
     {
-        var editor = await DocumentEditor.CreateAsync(context.Document, cancellationToken).ConfigureAwait(false);
-        var generator = editor.Generator;
-
-        var typeSymbol = editor.SemanticModel.Compilation.GetTypeByMetadataName("System.EventArgs");
-        if (typeSymbol is null)
-            return context.Document;
-
-        var newExpression = generator.TypeMemberAccessExpression(typeSymbol, nameof(EventArgs.Empty));
+        var editor = await DocumentEditor.CreateAsync(document, cancellationToken).ConfigureAwait(false);
+        var newExpression = editor.Generator.TypeMemberAccessExpression(eventArgsSymbol, nameof(EventArgs.Empty));
         editor.ReplaceNode(nodeToFix, newExpression);
         return editor.GetChangedDocument();
     }

tests/Meziantou.Analyzer.Test/Rules/EventsShouldHaveProperArgumentsAnalyzerTests.cs

@@ -405,4 +405,26 @@ void OnEvent()
 
         return test.RunAsync();
     }
+
+    [Fact]
+    public Task InvalidEventArgs_GenericEventArgs_NoCodeFix()
+    {
+        const string Source = """
+            using System;
+            delegate void CustomEventHandler<TEventArgs>(object sender, TEventArgs e) where TEventArgs : EventArgs;
+            class Test<TEventArgs> where TEventArgs : EventArgs
+            {
+                public event CustomEventHandler<TEventArgs> MyEvent;
+                void OnEvent()
+                {
+                    MyEvent?.Invoke(this, {|MA0093:null|});
+                }
+            }
+            """;
+        var test = CreateEventArgsFixTest();
+        test.TestCode = Source;
+        test.FixedCode = Source;
+
+        return test.RunAsync();
+    }
 }