Merge pull request #4285 from Sergio0694/bugfix/csharp8.0-mvvm-toolkit

Fix MVVM Toolkit build errors when using C# < 9.0

Author: michael-hawker

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

@@ -42,10 +42,11 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
-            // Validate the language version
-            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            // Validate the language version (this needs at least C# 8.0 due to static local functions being used).
+            // If a lower C# version is set, just skip the execution silently. The fallback path will be used just fine.
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp8 })
             {
-                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+                return;
             }
 
             // Get the symbol for the required attributes

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

@@ -39,10 +39,10 @@ public void Execute(GeneratorExecutionContext context)
                 return;
             }
 
-            // Validate the language version
-            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp9 })
+            // Like in the ObservableValidator.ValidateALlProperties generator, execution is skipped if C# >= 8.0 isn't available
+            if (context.ParseOptions is not CSharpParseOptions { LanguageVersion: >= LanguageVersion.CSharp8 })
             {
-                context.ReportDiagnostic(Diagnostic.Create(UnsupportedCSharpLanguageVersionError, null));
+                return;
             }
 
             // Get the symbol for the IRecipient<T> interface type

UnitTests/UnitTests.SourceGenerators/Test_SourceGeneratorsDiagnostics.cs

@@ -319,9 +319,9 @@ public partial class SampleViewModel : ObservableValidator
                 }
             }";
 
+            // This is explicitly allowed in C# < 9.0, as it doesn't use any new features
             VerifyGeneratedDiagnostics<ObservableValidatorValidateAllPropertiesGenerator>(
-                CSharpSyntaxTree.ParseText(source, CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_3)),
-                "MVVMTK0013");
+                CSharpSyntaxTree.ParseText(source, CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_3)));
         }
 
         [TestCategory("Mvvm")]
@@ -368,9 +368,9 @@ public void Receive(MyMessage message)
                 }
             }";
 
+            // This is explicitly allowed in C# < 9.0, as it doesn't use any new features
             VerifyGeneratedDiagnostics<IMessengerRegisterAllGenerator>(
-                CSharpSyntaxTree.ParseText(source, CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_3)),
-                "MVVMTK0013");
+                CSharpSyntaxTree.ParseText(source, CSharpParseOptions.Default.WithLanguageVersion(LanguageVersion.CSharp7_3)));
         }
 
         /// <summary>