Require that methods marked with GeneratedDllImport are in types that are marked partial and any parents of their containing types are also marked partial.

DllImportGenerator/DllImportGenerator.UnitTests/GeneratedDllImportAnalyzerTests.cs

@@ -139,5 +139,55 @@ partial class Test
 ";
             await VerifyCS.VerifyAnalyzerAsync(source);
         }
+
+        [Theory]
+        [InlineData("class")]
+        [InlineData("struct")]
+        [InlineData("record")]
+
+        public async Task NonPartialParentType_Diagnostic(string typeKind)
+        {
+            string source = $@"
+using System.Runtime.InteropServices;
+{typeKind} {{|#0:Test|}}
+{{
+    [GeneratedDllImport(""DoesNotExist"")]
+    static partial void {{|CS0751:Method2|}}();
+}}
+";
+
+            await VerifyCS.VerifyAnalyzerAsync(
+                source,
+                VerifyCS.Diagnostic(GeneratedDllImportContainingTypeMissingModifiers)
+                    .WithLocation(0)
+                    .WithArguments("Test"));
+        }
+
+        [Theory]
+        [InlineData("class")]
+        [InlineData("struct")]
+        [InlineData("record")]
+
+        public async Task NonPartialGrandparentType_Diagnostic(string typeKind)
+        {
+
+            string source = $@"
+using System.Runtime.InteropServices;
+{typeKind} {{|#0:Test|}}
+{{
+    partial class TestInner
+    {{
+        [GeneratedDllImport(""DoesNotExist"")]
+        static partial void Method2();
+    }}
+}}
+";
+
+            await VerifyCS.VerifyAnalyzerAsync(
+                source,
+                VerifyCS.Diagnostic(GeneratedDllImportContainingTypeMissingModifiers)
+                    .WithLocation(0)
+                    .WithArguments("Test"));
+        }
     }
 }

DllImportGenerator/DllImportGenerator/AnalyzerReleases.Unshipped.md

@@ -21,3 +21,4 @@ DLLIMPORTGENANALYZER013 | Usage            | Warning  | GeneratedDllImportMissin
 DLLIMPORTGENANALYZER014 | Usage            | Error    | RefValuePropertyUnsupported
 DLLIMPORTGENANALYZER015 | Interoperability | Disabled | ConvertToGeneratedDllImportAnalyzer
 DLLIMPORTGENANALYZER016 | Usage            | Error    | GenericTypeMustBeClosed
+DLLIMPORTGENANALYZER017 | Usage            | Warning  | GeneratedDllImportContainingTypeMissingModifiers

DllImportGenerator/DllImportGenerator/Analyzers/AnalyzerDiagnostics.cs

@@ -29,6 +29,7 @@ public static class Ids
 
             // GeneratedDllImport
             public const string GeneratedDllImportMissingRequiredModifiers = Prefix + "013";
+            public const string GeneratedDllImportContaiingTypeMissingRequiredModifiers = Prefix + "017";
 
             // Migration from DllImport to GeneratedDllImport
             public const string ConvertToGeneratedDllImport = Prefix + "015";

DllImportGenerator/DllImportGenerator/Analyzers/GeneratedDllImportAnalyzer.cs

@@ -25,7 +25,17 @@ public class GeneratedDllImportAnalyzer : DiagnosticAnalyzer
                 isEnabledByDefault: true,
                 description: GetResourceString(nameof(Resources.GeneratedDllImportMissingModifiersDescription)));
 
-        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(GeneratedDllImportMissingModifiers);
+        public readonly static DiagnosticDescriptor GeneratedDllImportContainingTypeMissingModifiers =
+            new DiagnosticDescriptor(
+                Ids.GeneratedDllImportContaiingTypeMissingRequiredModifiers,
+                GetResourceString(nameof(Resources.GeneratedDllImportContainingTypeMissingModifiersTitle)),
+                GetResourceString(nameof(Resources.GeneratedDllImportContainingTypeMissingModifiersMessage)),
+                Category,
+                DiagnosticSeverity.Warning,
+                isEnabledByDefault: true,
+                description: GetResourceString(nameof(Resources.GeneratedDllImportContainingTypeMissingModifiersDescription)));
+
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(GeneratedDllImportMissingModifiers, GeneratedDllImportContainingTypeMissingModifiers);
 
         public override void Initialize(AnalysisContext context)
         {
@@ -64,17 +74,27 @@ private static void AnalyzeSymbol(SymbolAnalysisContext context, INamedTypeSymbo
                 foreach (var reference in methodSymbol.DeclaringSyntaxReferences)
                 {
                     var syntax = reference.GetSyntax(context.CancellationToken);
-                    var methodSyntax = syntax as MethodDeclarationSyntax;
-                    if (methodSyntax == null)
-                        continue;
-
-                    if (!methodSyntax.Modifiers.Any(SyntaxKind.PartialKeyword))
+                    if (syntax is MethodDeclarationSyntax methodSyntax && !methodSyntax.Modifiers.Any(SyntaxKind.PartialKeyword))
                     {
                         // Must be marked partial
                         context.ReportDiagnostic(methodSymbol.CreateDiagnostic(GeneratedDllImportMissingModifiers, methodSymbol.Name));
                         break;
                     }
                 }
+
+                for (INamedTypeSymbol? typeSymbol = methodSymbol.ContainingType; typeSymbol is not null; typeSymbol = typeSymbol.ContainingType)
+                {
+                    foreach (var reference in typeSymbol.DeclaringSyntaxReferences)
+                    {
+                        var syntax = reference.GetSyntax(context.CancellationToken);
+                        if (syntax is TypeDeclarationSyntax typeSyntax && !typeSyntax.Modifiers.Any(SyntaxKind.PartialKeyword))
+                        {
+                            // Must be marked partial
+                            context.ReportDiagnostic(typeSymbol.CreateDiagnostic(GeneratedDllImportContainingTypeMissingModifiers, typeSymbol.Name));
+                            break;
+                        }
+                    }
+                }
             }
         }
     }

DllImportGenerator/DllImportGenerator/DllImportGenerator.cs

@@ -445,6 +445,15 @@ public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
                     return;
                 }
 
+                // Verify that the types the method is declared in are marked partial.
+                for (SyntaxNode? parentNode = methodSyntax.Parent; parentNode is TypeDeclarationSyntax typeDecl; parentNode = parentNode.Parent)
+                {
+                    if (!typeDecl.Modifiers.Any(SyntaxKind.PartialKeyword))
+                    {
+                        return;
+                    }
+                }
+
                 // Check if the method is marked with the GeneratedDllImport attribute.
                 foreach (AttributeListSyntax listSyntax in methodSyntax.AttributeLists)
                 {

DllImportGenerator/DllImportGenerator/Resources.resx

@@ -181,6 +181,15 @@
   <data name="CustomTypeMarshallingNativeToManagedUnsupported" xml:space="preserve">
     <value>The specified parameter needs to be marshalled from native to managed, but the native type '{0}' does not support it.</value>
   </data>
+  <data name="GeneratedDllImportContainingTypeMissingModifiersDescription" xml:space="preserve">
+    <value>Types that contain methods marked with 'GeneratedDllImportAttribute' must be 'partial'. P/Invoke source generation will ignore methods contained within non-partial types.</value>
+  </data>
+  <data name="GeneratedDllImportContainingTypeMissingModifiersMessage" xml:space="preserve">
+    <value>Type '{0}' contains methods marked with 'GeneratedDllImportAttribute' and should be 'partial'. P/Invoke source generation will ignore methods contained within non-partial types.</value>
+  </data>
+  <data name="GeneratedDllImportContainingTypeMissingModifiersTitle" xml:space="preserve">
+    <value>Types that contain methods marked with 'GeneratedDllImportAttribute' must be 'partial'.</value>
+  </data>
   <data name="GeneratedDllImportMissingModifiersDescription" xml:space="preserve">
     <value>Methods marked with 'GeneratedDllImportAttribute' should be 'static' and 'partial'. P/Invoke source generation will ignore methods that are not 'static' and 'partial'.</value>
   </data>