Add CS1066 suppressor for MCP server methods with optional parameters (#1110)

src/ModelContextProtocol.Analyzers/CS1066Suppressor.cs

@@ -0,0 +1,148 @@
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Threading;
+
+namespace ModelContextProtocol.Analyzers;
+
+/// <summary>
+/// Suppresses CS1066 warnings for MCP server methods that have optional parameters.
+/// </summary>
+/// <remarks>
+/// <para>
+/// CS1066 is issued when a partial method's implementing declaration has default parameter values.
+/// For partial methods, only the defining declaration's defaults are used by callers,
+/// making the implementing declaration's defaults redundant.
+/// </para>
+/// <para>
+/// However, for MCP tool, prompt, and resource methods, users often want to specify default values
+/// in their implementing declaration for documentation purposes. The XmlToDescriptionGenerator
+/// automatically copies these defaults to the generated defining declaration, making them functional.
+/// </para>
+/// <para>
+/// This suppressor suppresses CS1066 for methods marked with [McpServerTool], [McpServerPrompt],
+/// or [McpServerResource] attributes, allowing users to specify defaults in their code without warnings.
+/// </para>
+/// </remarks>
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class CS1066Suppressor : DiagnosticSuppressor
+{
+    private static readonly SuppressionDescriptor McpToolSuppression = new(
+        id: "MCP_CS1066_TOOL",
+        suppressedDiagnosticId: "CS1066",
+        justification: "Default values on MCP tool method implementing declarations are copied to the generated defining declaration by the source generator.");
+
+    private static readonly SuppressionDescriptor McpPromptSuppression = new(
+        id: "MCP_CS1066_PROMPT",
+        suppressedDiagnosticId: "CS1066",
+        justification: "Default values on MCP prompt method implementing declarations are copied to the generated defining declaration by the source generator.");
+
+    private static readonly SuppressionDescriptor McpResourceSuppression = new(
+        id: "MCP_CS1066_RESOURCE",
+        suppressedDiagnosticId: "CS1066",
+        justification: "Default values on MCP resource method implementing declarations are copied to the generated defining declaration by the source generator.");
+
+    /// <inheritdoc/>
+    public override ImmutableArray<SuppressionDescriptor> SupportedSuppressions =>
+        ImmutableArray.Create(McpToolSuppression, McpPromptSuppression, McpResourceSuppression);
+
+    /// <inheritdoc/>
+    public override void ReportSuppressions(SuppressionAnalysisContext context)
+    {
+        // Cache semantic models and attribute symbols per syntax tree/compilation to avoid redundant calls
+        Dictionary<SyntaxTree, SemanticModel>? semanticModelCache = null;
+        INamedTypeSymbol? mcpToolAttribute = null;
+        INamedTypeSymbol? mcpPromptAttribute = null;
+        INamedTypeSymbol? mcpResourceAttribute = null;
+        bool attributesResolved = false;
+
+        foreach (Diagnostic diagnostic in context.ReportedDiagnostics)
+        {
+            Location? location = diagnostic.Location;
+            SyntaxTree? tree = location.SourceTree;
+            if (tree is null)
+            {
+                continue;
+            }
+
+            SyntaxNode root = tree.GetRoot(context.CancellationToken);
+            SyntaxNode? node = root.FindNode(location.SourceSpan);
+
+            // Find the containing method declaration
+            MethodDeclarationSyntax? method = node.FirstAncestorOrSelf<MethodDeclarationSyntax>();
+            if (method is null)
+            {
+                continue;
+            }
+
+            // Get or cache the semantic model for this tree
+            semanticModelCache ??= new Dictionary<SyntaxTree, SemanticModel>();
+            if (!semanticModelCache.TryGetValue(tree, out SemanticModel? semanticModel))
+            {
+                semanticModel = context.GetSemanticModel(tree);
+                semanticModelCache[tree] = semanticModel;
+            }
+
+            // Resolve attribute symbols once per compilation
+            if (!attributesResolved)
+            {
+                mcpToolAttribute = semanticModel.Compilation.GetTypeByMetadataName(McpAttributeNames.McpServerToolAttribute);
+                mcpPromptAttribute = semanticModel.Compilation.GetTypeByMetadataName(McpAttributeNames.McpServerPromptAttribute);
+                mcpResourceAttribute = semanticModel.Compilation.GetTypeByMetadataName(McpAttributeNames.McpServerResourceAttribute);
+                attributesResolved = true;
+            }
+
+            // Check for MCP attributes
+            SuppressionDescriptor? suppression = GetSuppressionForMethod(method, semanticModel, mcpToolAttribute, mcpPromptAttribute, mcpResourceAttribute, context.CancellationToken);
+            if (suppression is not null)
+            {
+                context.ReportSuppression(Suppression.Create(suppression, diagnostic));
+            }
+        }
+    }
+
+    private static SuppressionDescriptor? GetSuppressionForMethod(
+        MethodDeclarationSyntax method,
+        SemanticModel semanticModel,
+        INamedTypeSymbol? mcpToolAttribute,
+        INamedTypeSymbol? mcpPromptAttribute,
+        INamedTypeSymbol? mcpResourceAttribute,
+        CancellationToken cancellationToken)
+    {
+        IMethodSymbol? methodSymbol = semanticModel.GetDeclaredSymbol(method, cancellationToken);
+
+        if (methodSymbol is null)
+        {
+            return null;
+        }
+
+        foreach (AttributeData attribute in methodSymbol.GetAttributes())
+        {
+            INamedTypeSymbol? attributeClass = attribute.AttributeClass;
+            if (attributeClass is null)
+            {
+                continue;
+            }
+
+            if (mcpToolAttribute is not null && SymbolEqualityComparer.Default.Equals(attributeClass, mcpToolAttribute))
+            {
+                return McpToolSuppression;
+            }
+
+            if (mcpPromptAttribute is not null && SymbolEqualityComparer.Default.Equals(attributeClass, mcpPromptAttribute))
+            {
+                return McpPromptSuppression;
+            }
+
+            if (mcpResourceAttribute is not null && SymbolEqualityComparer.Default.Equals(attributeClass, mcpResourceAttribute))
+            {
+                return McpResourceSuppression;
+            }
+        }
+
+        return null;
+    }
+}

src/ModelContextProtocol.Analyzers/McpAttributeNames.cs

@@ -0,0 +1,12 @@
+namespace ModelContextProtocol.Analyzers;
+
+/// <summary>
+/// Contains the fully qualified metadata names for MCP server attributes.
+/// </summary>
+internal static class McpAttributeNames
+{
+    public const string McpServerToolAttribute = "ModelContextProtocol.Server.McpServerToolAttribute";
+    public const string McpServerPromptAttribute = "ModelContextProtocol.Server.McpServerPromptAttribute";
+    public const string McpServerResourceAttribute = "ModelContextProtocol.Server.McpServerResourceAttribute";
+    public const string DescriptionAttribute = "System.ComponentModel.DescriptionAttribute";
+}

src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs

@@ -18,18 +18,14 @@ namespace ModelContextProtocol.Analyzers;
 public sealed class XmlToDescriptionGenerator : IIncrementalGenerator
 {
     private const string GeneratedFileName = "ModelContextProtocol.Descriptions.g.cs";
-    private const string McpServerToolAttributeName = "ModelContextProtocol.Server.McpServerToolAttribute";
-    private const string McpServerPromptAttributeName = "ModelContextProtocol.Server.McpServerPromptAttribute";
-    private const string McpServerResourceAttributeName = "ModelContextProtocol.Server.McpServerResourceAttribute";
-    private const string DescriptionAttributeName = "System.ComponentModel.DescriptionAttribute";
 
     public void Initialize(IncrementalGeneratorInitializationContext context)
     {
         // Extract method information for all MCP tools, prompts, and resources.
         // The transform extracts all necessary data upfront so the output doesn't depend on the compilation.
-        var allMethods = CreateProviderForAttribute(context, McpServerToolAttributeName).Collect()
-            .Combine(CreateProviderForAttribute(context, McpServerPromptAttributeName).Collect())
-            .Combine(CreateProviderForAttribute(context, McpServerResourceAttributeName).Collect())
+        var allMethods = CreateProviderForAttribute(context, McpAttributeNames.McpServerToolAttribute).Collect()
+            .Combine(CreateProviderForAttribute(context, McpAttributeNames.McpServerPromptAttribute).Collect())
+            .Combine(CreateProviderForAttribute(context, McpAttributeNames.McpServerResourceAttribute).Collect())
             .Select(static (tuple, _) =>
             {
                 var ((tools, prompts), resources) = tuple;
@@ -84,7 +80,7 @@ public sealed class XmlToDescriptionGenerator : IIncrementalGenerator
         Compilation compilation)
     {
         bool isPartial = methodDeclaration.Modifiers.Any(SyntaxKind.PartialKeyword);
-        var descriptionAttribute = compilation.GetTypeByMetadataName(DescriptionAttributeName);
+        var descriptionAttribute = compilation.GetTypeByMetadataName(McpAttributeNames.DescriptionAttribute);
 
         // Try to extract XML documentation
         var (xmlDocs, hasInvalidXml) = TryExtractXmlDocumentation(methodSymbol);