Added diagnostic for unsupported C# language version

Author: Sergio0694

Microsoft.Toolkit.Mvvm.SourceGenerators/AnalyzerReleases.Unshipped.md

@@ -17,3 +17,4 @@ MVVMTK0009 | Microsoft.Toolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator
 MVVMTK0010 | Microsoft.Toolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/error
 MVVMTK0011 | Microsoft.Toolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/error
 MVVMTK0012 | Microsoft.Toolkit.Mvvm.SourceGenerators.ICommandGenerator | Error | See https://aka.ms/mvvmtoolkit/error
+MVVMTK0013 | Microsoft.CodeAnalysis.CSharp.CSharpParseOptions | Error | See https://aka.ms/mvvmtoolkit/error

Microsoft.Toolkit.Mvvm.SourceGenerators/ComponentModel/ObservablePropertyGenerator.cs

@@ -41,6 +41,15 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Validate the language version. Note that we're emitting this diagnostic in each generator (excluding the one
+            // only emitting the nullability annotation attributes if missing) so that the diagnostic is emitted only when
+            // users are using one of these generators, and not by default as soon as they add a reference to the MVVM Toolkit.
+            // This ensures that users not using any of the source generators won't be broken when upgrading to this new version.
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            {
+                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+            }
+
             // Sets of discovered property names
             HashSet<string>
                 propertyChangedNames = new(),

Microsoft.Toolkit.Mvvm.SourceGenerators/ComponentModel/ObservableValidatorValidateAllPropertiesGenerator.cs

@@ -14,6 +14,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.Toolkit.Mvvm.SourceGenerators.Extensions;
 using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
+using static Microsoft.Toolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
 
 namespace Microsoft.Toolkit.Mvvm.SourceGenerators
 {
@@ -39,6 +40,12 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Validate the language version
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            {
+                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+            }
+
             // Get the symbol for the ValidationAttribute type
             INamedTypeSymbol validationSymbol = context.Compilation.GetTypeByMetadataName("System.ComponentModel.DataAnnotations.ValidationAttribute")!;
 

Microsoft.Toolkit.Mvvm.SourceGenerators/ComponentModel/TransitiveMembersGenerator.cs

@@ -18,6 +18,7 @@
 using Microsoft.Toolkit.Mvvm.SourceGenerators.Extensions;
 using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
 using static Microsoft.CodeAnalysis.SymbolDisplayTypeQualificationStyle;
+using static Microsoft.Toolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
 
 namespace Microsoft.Toolkit.Mvvm.SourceGenerators
 {
@@ -67,6 +68,12 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Validate the language version
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            {
+                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+            }
+
             // Load the syntax tree with the members to generate
             SyntaxTree sourceSyntaxTree = LoadSourceSyntaxTree();
 

Microsoft.Toolkit.Mvvm.SourceGenerators/Diagnostics/DiagnosticDescriptors.cs

@@ -4,6 +4,7 @@
 
 using System.ComponentModel;
 using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
 
 namespace Microsoft.Toolkit.Mvvm.SourceGenerators.Diagnostics
 {
@@ -201,7 +202,23 @@ internal static class DiagnosticDescriptors
             category: typeof(ICommandGenerator).FullName,
             defaultSeverity: DiagnosticSeverity.Error,
             isEnabledByDefault: true,
-            description: $"Cannot apply [ICommand] to methods with a signature that doesn't match any of the existing relay command types.",
+            description: "Cannot apply [ICommand] to methods with a signature that doesn't match any of the existing relay command types.",
+            helpLinkUri: "https://aka.ms/mvvmtoolkit");
+
+        /// <summary>
+        /// Gets a <see cref="DiagnosticDescriptor"/> indicating when an unsupported C# language version is being used.
+        /// <para>
+        /// Format: <c>"The method {0}.{1} cannot be used to generate a command property, as its signature isn't compatible with any of the existing relay command types"</c>.
+        /// </para>
+        /// </summary>
+        public static readonly DiagnosticDescriptor UnsupportedCSharpLanguageVersionError = new(
+            id: "MVVMTK0013",
+            title: "Unsupported C# language version",
+            messageFormat: "The source generator features from the MVVM Toolkit require consuming projects to set the C# language version to at least C# 9.0",
+            category: typeof(CSharpParseOptions).FullName,
+            defaultSeverity: DiagnosticSeverity.Error,
+            isEnabledByDefault: true,
+            description: "The source generator features from the MVVM Toolkit require consuming projects to set the C# language version to at least C# 9.0. Make sure to add <LangVersion>9.0</LangVersion> (or above) to your .csproj file.",
             helpLinkUri: "https://aka.ms/mvvmtoolkit");
     }
 }

Microsoft.Toolkit.Mvvm.SourceGenerators/Input/ICommandGenerator.cs

@@ -43,6 +43,12 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Validate the language version
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            {
+                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+            }
+
             foreach (var items in syntaxReceiver.GatheredInfo.GroupBy<SyntaxReceiver.Item, INamedTypeSymbol>(static item => item.MethodSymbol.ContainingType, SymbolEqualityComparer.Default))
             {
                 if (items.Key.DeclaringSyntaxReferences.Length > 0 &&

Microsoft.Toolkit.Mvvm.SourceGenerators/Messaging/IMessengerRegisterAllGenerator.cs

@@ -13,6 +13,7 @@
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.Toolkit.Mvvm.SourceGenerators.Extensions;
 using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
+using static Microsoft.Toolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
 
 namespace Microsoft.Toolkit.Mvvm.SourceGenerators
 {
@@ -38,6 +39,12 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
+            // Validate the language version
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            {
+                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+            }
+
             // Get the symbol for the IRecipient<T> interface type
             INamedTypeSymbol iRecipientSymbol = context.Compilation.GetTypeByMetadataName("Microsoft.Toolkit.Mvvm.Messaging.IRecipient`1")!;