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Add CS1066 suppressor for MCP server methods with optional parameters (#1110)
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‎src/ModelContextProtocol.Analyzers/CS1066Suppressor.cs‎

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

‎src/ModelContextProtocol.Analyzers/McpAttributeNames.cs‎

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

‎src/ModelContextProtocol.Analyzers/XmlToDescriptionGenerator.cs‎

Lines changed: 4 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -18,18 +18,14 @@ namespace ModelContextProtocol.Analyzers;
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public sealed class XmlToDescriptionGenerator : IIncrementalGenerator
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{
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private const string GeneratedFileName = "ModelContextProtocol.Descriptions.g.cs";
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private const string McpServerToolAttributeName = "ModelContextProtocol.Server.McpServerToolAttribute";
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private const string McpServerPromptAttributeName = "ModelContextProtocol.Server.McpServerPromptAttribute";
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private const string McpServerResourceAttributeName = "ModelContextProtocol.Server.McpServerResourceAttribute";
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private const string DescriptionAttributeName = "System.ComponentModel.DescriptionAttribute";
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public void Initialize(IncrementalGeneratorInitializationContext context)
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{
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// Extract method information for all MCP tools, prompts, and resources.
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// The transform extracts all necessary data upfront so the output doesn't depend on the compilation.
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var allMethods = CreateProviderForAttribute(context, McpServerToolAttributeName).Collect()
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.Combine(CreateProviderForAttribute(context, McpServerPromptAttributeName).Collect())
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.Combine(CreateProviderForAttribute(context, McpServerResourceAttributeName).Collect())
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var allMethods = CreateProviderForAttribute(context, McpAttributeNames.McpServerToolAttribute).Collect()
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.Combine(CreateProviderForAttribute(context, McpAttributeNames.McpServerPromptAttribute).Collect())
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.Combine(CreateProviderForAttribute(context, McpAttributeNames.McpServerResourceAttribute).Collect())
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.Select(static (tuple, _) =>
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{
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var ((tools, prompts), resources) = tuple;
@@ -84,7 +80,7 @@ private static MethodToGenerate ExtractMethodInfo(
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Compilation compilation)
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{
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bool isPartial = methodDeclaration.Modifiers.Any(SyntaxKind.PartialKeyword);
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var descriptionAttribute = compilation.GetTypeByMetadataName(DescriptionAttributeName);
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var descriptionAttribute = compilation.GetTypeByMetadataName(McpAttributeNames.DescriptionAttribute);
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// Try to extract XML documentation
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var (xmlDocs, hasInvalidXml) = TryExtractXmlDocumentation(methodSymbol);

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