Suppress CS3016 on internal COM wrappers without [CLSCompliant(false)] (#1703)

src/Microsoft.Windows.CsWin32/Generator.Com.cs

@@ -769,20 +769,6 @@ static ExpressionSyntax ThisPointer(PointerTypeSyntax? typedPointer = null)
             iface = iface.AddAttributeLists(AttributeList(GUID(DecodeGuidFromAttribute(guidAttribute.Value))));
         }
 
-        // CS3016 ("Arrays as attribute arguments is not CLS-compliant") fires under
-        // [assembly: CLSCompliant(true)] on the CCW thunks we emit with
-        // [UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvStdcall) })]. CCW thunks are
-        // only emitted when canUseUnmanagedCallersOnlyAttribute is true (.NET 5+); on
-        // net472 / netstandard2.0 the generator produces no array-valued attribute arguments and
-        // [CLSCompliant(false)] is unnecessary. The attribute is also a no-op on public types
-        // (which the consumer's CLS surface owns), so we only emit it on internal struct
-        // wrappers that we know carry the array-valued attribute.
-        // See https://github.com/microsoft/CsWin32/issues/1703.
-        if (ccwThisParameter is not null && this.Visibility == SyntaxKind.InternalKeyword)
-        {
-            iface = iface.AddAttributeLists(AttributeList(CLSCompliantFalse()));
-        }
-
         if (this.GetSupportedOSPlatformAttribute(typeDef.GetCustomAttributes()) is AttributeSyntax supportedOSPlatformAttribute)
         {
             iface = iface.AddAttributeLists(AttributeList(supportedOSPlatformAttribute));

src/Microsoft.Windows.CsWin32/Generator.Invariants.cs

@@ -296,16 +296,40 @@ public partial class Generator
         "CS0436", // conflicts with the imported type (InternalsVisibleTo between two projects that both use CsWin32)
         "CS8981", // The type name only contains lower-cased ascii characters
         "SYSLIB1092", // The return value in the managed definition will be converted to an 'out' parameter when calling the unmanaged COM method
-        "CS3021", // Type does not need a CLSCompliant attribute because the assembly does not have a CLSCompliant attribute (fires on generated COM struct wrappers that we mark [CLSCompliant(false)] to silence CS3016 — see https://github.com/microsoft/CsWin32/issues/1703)
-        "CS3019", // CLS compliance checking will not be performed on '...' because it is not visible from outside this assembly (same root cause: our internal COM struct wrappers are marked [CLSCompliant(false)] to silence CS3016)
     };
 
+    /// <summary>
+    /// The leading trivia for every generated file: the auto-generated banner plus a <c>#pragma warning disable</c> for the
+    /// warnings CsWin32-generated code is expected to trip. Used for <em>public</em> projections.
+    /// </summary>
     private static readonly SyntaxTriviaList FileHeader = ParseLeadingTrivia(AutoGeneratedHeader).Add(
         Trivia(PragmaWarningDirectiveTrivia(
             disableOrRestoreKeyword: TokenWithSpace(SyntaxKind.DisableKeyword),
             errorCodes: [.. WarningsToSuppressInGeneratedCode.Select(IdentifierName)],
             isActive: true)));
 
+    /// <summary>
+    /// Same as <see cref="FileHeader"/>, but additionally suppresses CS3016 ("Arrays as attribute arguments is not
+    /// CLS-compliant"). Used for <em>internal</em> projections (the default).
+    /// </summary>
+    /// <remarks>
+    /// CS3016 fires under <c>[assembly: CLSCompliant(true)]</c> on the CCW thunks we emit with
+    /// <c>[UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvStdcall) })]</c> — a Roslyn limitation that reports the
+    /// diagnostic even for non-visible members (dotnet/roslyn#68526). The thunks only exist on .NET 5+ (net472 /
+    /// netstandard2.0 never trip it). Suppressing it via this header pragma — at the thunk site, always in our generated file —
+    /// masks no user-authored CS3016. We deliberately do NOT instead stamp <c>[CLSCompliant(false)]</c> on the struct: that
+    /// type-level attribute induces CS3019/CS3021, which Roslyn reports against the type symbol and therefore leak onto any
+    /// consumer-authored partial of the struct (e.g. a friendly-helper partial) where the generated pragma cannot reach.
+    /// We use this header only for internal projections; for public ones the consumer owns the CLS-compliance contract of
+    /// their public surface (which inherently exposes other non-CLS COM types), so CsWin32 must not unilaterally silence it.
+    /// See https://github.com/microsoft/CsWin32/issues/1703.
+    /// </remarks>
+    private static readonly SyntaxTriviaList FileHeaderWithClsArrayAttributeSuppression = ParseLeadingTrivia(AutoGeneratedHeader).Add(
+        Trivia(PragmaWarningDirectiveTrivia(
+            disableOrRestoreKeyword: TokenWithSpace(SyntaxKind.DisableKeyword),
+            errorCodes: [.. WarningsToSuppressInGeneratedCode.Append("CS3016").Select(IdentifierName)],
+            isActive: true)));
+
     private static readonly AttributeSyntax InAttributeSyntax = Attribute(IdentifierName("In")).WithArgumentList(null);
     private static readonly AttributeSyntax OutAttributeSyntax = Attribute(IdentifierName("Out")).WithArgumentList(null);
     private static readonly AttributeSyntax OptionalAttributeSyntax = Attribute(IdentifierName("Optional")).WithArgumentList(null);

src/Microsoft.Windows.CsWin32/Generator.cs

@@ -42,6 +42,7 @@ public partial class Generator : IGenerator, IDisposable
     private readonly GeneratorOptions options;
     private readonly CSharpCompilation? compilation;
     private readonly CSharpParseOptions? parseOptions;
+    private readonly SyntaxTriviaList fileHeader;
     private readonly bool comIIDInterfacePredefined;
     private readonly bool getDelegateForFunctionPointerGenericExists;
     private readonly GeneratedCode committedCode = new();
@@ -107,6 +108,10 @@ public Generator(string metadataLibraryPath, Docs? docs, IEnumerable<string> add
         this.parseOptions = parseOptions;
         this.volatileCode = new(this.committedCode);
 
+        // Suppress CS3016 (the CLS array-attribute-argument warning our CCW thunks trip) in the generated file header,
+        // but only for internal projections. For public projections the consumer owns their public surface's CLS contract.
+        this.fileHeader = this.options.Public ? FileHeader : FileHeaderWithClsArrayAttributeSuppression;
+
         // UnscopedRefAttribute may be emitted to work on downlevel *runtimes*, but we can't use it
         // on downlevel *compilers*. Only .NET 8+ SDK compilers support it. Since we cannot detect
         // compiler version, we use language version instead.
@@ -854,7 +859,7 @@ .. this.committedCode.GeneratedTopLevelTypes
             CompilationUnitSyntax? compilationUnit = ((CompilationUnitSyntax)kv.Value
                 .AddUsings(usingDirectives.ToArray())
                 .Accept(new WhitespaceRewriter())!)
-                .WithLeadingTrivia(FileHeader);
+                .WithLeadingTrivia(this.fileHeader);
 
             lock (normalizedResults)
             {
@@ -871,7 +876,7 @@ .. this.committedCode.GeneratedTopLevelTypes
             }
             else
             {
-                normalizedResults.Add(string.Format(CultureInfo.InvariantCulture, FilenamePattern, "CsWin32Stamp"), CompilationUnit().AddAttributeLists(CsWin32StampAttribute).WithLeadingTrivia(FileHeader));
+                normalizedResults.Add(string.Format(CultureInfo.InvariantCulture, FilenamePattern, "CsWin32Stamp"), CompilationUnit().AddAttributeLists(CsWin32StampAttribute).WithLeadingTrivia(this.fileHeader));
             }
         }
 

src/Microsoft.Windows.CsWin32/SimpleSyntaxFactory.cs

@@ -197,12 +197,6 @@ internal static AttributeSyntax GUID(Guid guid)
             AttributeArgument(LiteralExpression(SyntaxKind.StringLiteralExpression, Literal(guid.ToString().ToUpperInvariant()))));
     }
 
-    internal static AttributeSyntax CLSCompliantFalse()
-    {
-        return Attribute(IdentifierName("CLSCompliant")).AddArgumentListArguments(
-            AttributeArgument(LiteralExpression(SyntaxKind.FalseLiteralExpression)));
-    }
-
     internal static AttributeSyntax InterfaceType(ComInterfaceType interfaceType)
     {
         return Attribute(IdentifierName("InterfaceType")).AddArgumentListArguments(