Add diagnostics for invalid [ObservableRecipient] target type

Author: Sergio0694

CommunityToolkit.Mvvm.SourceGenerators/AnalyzerReleases.Unshipped.md

@@ -25,3 +25,4 @@ MVVMTK0017 | CommunityToolkit.Mvvm.SourceGenerators.INotifyPropertyChangedGenera
 MVVMTK0018 | CommunityToolkit.Mvvm.SourceGenerators.ObservableObjectGenerator | Error | See https://aka.ms/mvvmtoolkit/error
 MVVMTK0019 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/error
 MVVMTK0020 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/error
+MVVMTK0021 | CommunityToolkit.Mvvm.SourceGenerators.ObservableRecipientGenerator | Error | See https://aka.ms/mvvmtoolkit/error

CommunityToolkit.Mvvm.SourceGenerators/ComponentModel/ObservableRecipientGenerator.cs

@@ -79,6 +79,16 @@ protected override bool ValidateTargetType(INamedTypeSymbol typeSymbol, Observab
             return false;
         }
 
+        // Check if the type already inherits [ObservableRecipient]
+        if (typeSymbol.InheritsAttributeWithFullyQualifiedName("global::CommunityToolkit.Mvvm.ComponentModel.ObservableRecipientAttribute"))
+        {
+            builder.Add(InvalidAttributeCombinationForObservableRecipientAttributeError, typeSymbol, typeSymbol);
+
+            diagnostics = builder.ToImmutable();
+
+            return false;
+        }
+
         // In order to use [ObservableRecipient], the target type needs to inherit from ObservableObject,
         // or be annotated with [ObservableObject] or [INotifyPropertyChanged] (with additional helpers).
         if (!typeSymbol.InheritsFromFullyQualifiedName("global::CommunityToolkit.Mvvm.ComponentModel.ObservableObject") &&

CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/DiagnosticDescriptors.cs

@@ -331,4 +331,20 @@ internal static class DiagnosticDescriptors
         isEnabledByDefault: true,
         description: "Fields not annotated with [ObservableProperty] cannot use [AlsoNotifyChangeFor] and [AlsoNotifyCanExecuteFor].",
         helpLinkUri: "https://aka.ms/mvvmtoolkit");
+
+    /// <summary>
+    /// Gets a <see cref="DiagnosticDescriptor"/> indicating when <c>[ObservableRecipient]</c> is applied to a type with an attribute already.
+    /// <para>
+    /// Format: <c>"Cannot apply [ObservableRecipient] to type {0}, as it already inherits this attribute from a base type"</c>.
+    /// </para>
+    /// </summary>
+    public static readonly DiagnosticDescriptor InvalidAttributeCombinationForObservableRecipientAttributeError = new DiagnosticDescriptor(
+        id: "MVVMTK0021",
+        title: "Invalid target type for [ObservableRecipient]",
+        messageFormat: "Cannot apply [ObservableRecipient] to type {0}, as it already inherits this attribute from a base type",
+        category: typeof(ObservableRecipientGenerator).FullName,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true,
+        description: "Cannot apply [ObservableRecipient] to a type that already inherits this attribute from a base type.",
+        helpLinkUri: "https://aka.ms/mvvmtoolkit");
 }

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

@@ -998,6 +998,28 @@ public partial class MyViewModel
         VerifyGeneratedDiagnostics<ObservablePropertyGenerator>(source, "MVVMTK0020");
     }
 
+    [TestMethod]
+    public void InvalidAttributeCombinationForObservableRecipientAttributeError()
+    {
+        string source = @"
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                [ObservableRecipient]
+                public partial class A : ObservableObject
+                {
+                }
+
+                [ObservableRecipient]
+                public partial class B : A
+                {
+                }
+            }";
+
+        VerifyGeneratedDiagnostics<ObservableRecipientGenerator>(source, "MVVMTK0021");
+    }
+
     /// <summary>
     /// Verifies the output of a source generator.
     /// </summary>