Merge pull request #539 from CommunityToolkit/dev/skip-diagnostics-within-constructors

Skip field reference diagnostics from constructors

Author: Sergio0694

src/CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/Analyzers/FieldReferenceForObservablePropertyFieldAnalyzer.cs

@@ -28,7 +28,25 @@ public override void Initialize(AnalysisContext context)
         context.RegisterOperationAction(static context =>
         {
             // We're only looking for references to fields that could potentially be observable properties
-            if (context.Operation is not IFieldReferenceOperation { Field: IFieldSymbol { IsStatic: false, IsConst: false, IsImplicitlyDeclared: false, ContainingType: INamedTypeSymbol } fieldSymbol })
+            if (context.Operation is not IFieldReferenceOperation
+                {
+                    Field: IFieldSymbol { IsStatic: false, IsConst: false, IsImplicitlyDeclared: false, ContainingType: INamedTypeSymbol } fieldSymbol,
+                    Instance.Type: ITypeSymbol typeSymbol
+                })
+            {
+                return;
+            }
+
+            // Special case field references from within a constructor and don't ever emit warnings for them. The point of this
+            // analyzer is to prevent mistakes when users assign a field instead of a property and then get confused when the
+            // property changed event is not raised. But this would never be the case from a constructur anyway, given that
+            // no handler for that event would possibly be present. Suppressing warnings in this cases though will help to
+            // avoid scenarios where people get nullability warnings they cannot suppress, in case they were pushed by the
+            // analyzer in the MVVM Toolkit to not assign a field marked with a non-nullable reference type. Ideally this
+            // would be solved by habing the generated setter be marked with [MemberNotNullIfNotNull("field", "value")],
+            // but such an annotation does not currently exist.
+            if (context.ContainingSymbol is IMethodSymbol { MethodKind: MethodKind.Constructor, ContainingType: INamedTypeSymbol instanceType } &&
+                SymbolEqualityComparer.Default.Equals(instanceType, typeSymbol))
             {
                 return;
             }

tests/CommunityToolkit.Mvvm.SourceGenerators.UnitTests/Test_SourceGeneratorsDiagnostics.cs

@@ -1602,6 +1602,29 @@ void M()
         await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<FieldReferenceForObservablePropertyFieldAnalyzer>(source, LanguageVersion.CSharp8);
     }
 
+    [TestMethod]
+    public async Task FieldReferenceToFieldWithObservablePropertyAttribute_DoesNotEmitWarningFromWithinConstructor()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            partial class MyViewModel : ObservableObject
+            {
+                [ObservableProperty]
+                int number;
+
+                public MyViewModel()
+                {
+                    number = 42;
+                    var temp = number;
+                    ref var x = ref number;
+                }
+            }
+            """;
+
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<FieldReferenceForObservablePropertyFieldAnalyzer>(source, LanguageVersion.CSharp8);
+    }
+
     /// <summary>
     /// Verifies the diagnostic errors for a given analyzer, and that all available source generators can run successfully with the input source (including subsequent compilation).
     /// </summary>