Address asymmetry in McpServerHandlers/McpClientHandlers and make all filter properties settable (#1337)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Client/McpClientHandlers.cs

@@ -22,7 +22,7 @@ namespace ModelContextProtocol.Client;
 /// is done based on an ordinal, case-sensitive string comparison.
 /// </para>
 /// </remarks>
-public class McpClientHandlers
+public sealed class McpClientHandlers
 {
     /// <summary>Gets or sets notification handlers to register with the client.</summary>
     /// <remarks>

src/ModelContextProtocol.Core/Server/McpMessageFilters.cs

@@ -8,7 +8,7 @@ namespace ModelContextProtocol.Server;
 public sealed class McpMessageFilters
 {
     /// <summary>
-    /// Gets the filters for all incoming JSON-RPC messages.
+    /// Gets or sets the filters for all incoming JSON-RPC messages.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -21,10 +21,18 @@ public sealed class McpMessageFilters
     /// the next handler in the pipeline, the default handlers will not be executed.
     /// </para>
     /// </remarks>
-    public IList<McpMessageFilter> IncomingFilters { get; } = [];
+    public IList<McpMessageFilter> IncomingFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for all outgoing JSON-RPC messages.
+    /// Gets or sets the filters for all outgoing JSON-RPC messages.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -38,5 +46,13 @@ public sealed class McpMessageFilters
     /// server-to-client messages.
     /// </para>
     /// </remarks>
-    public IList<McpMessageFilter> OutgoingFilters { get; } = [];
+    public IList<McpMessageFilter> OutgoingFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 }

src/ModelContextProtocol.Core/Server/McpRequestFilters.cs

@@ -8,7 +8,7 @@ namespace ModelContextProtocol.Server;
 public sealed class McpRequestFilters
 {
     /// <summary>
-    /// Gets the filters for the list-tools handler pipeline.
+    /// Gets or sets the filters for the list-tools handler pipeline.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -22,20 +22,36 @@ public sealed class McpRequestFilters
     /// Tools from both sources will be combined when returning results to clients.
     /// </para>
     /// </remarks>
-    public IList<McpRequestFilter<ListToolsRequestParams, ListToolsResult>> ListToolsFilters { get; } = [];
+    public IList<McpRequestFilter<ListToolsRequestParams, ListToolsResult>> ListToolsFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the call-tool handler pipeline.
+    /// Gets or sets the filters for the call-tool handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that are invoked when a client makes a call to a tool that isn't found in the <see cref="McpServerTool"/> collection.
     /// The filters can modify, log, or perform additional operations on requests and responses for
     /// <see cref="RequestMethods.ToolsCall"/> requests. The handler should implement logic to execute the requested tool and return appropriate results.
     /// </remarks>
-    public IList<McpRequestFilter<CallToolRequestParams, CallToolResult>> CallToolFilters { get; } = [];
+    public IList<McpRequestFilter<CallToolRequestParams, CallToolResult>> CallToolFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the list-prompts handler pipeline.
+    /// Gets or sets the filters for the list-prompts handler pipeline.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -49,20 +65,36 @@ public sealed class McpRequestFilters
     /// Prompts from both sources will be combined when returning results to clients.
     /// </para>
     /// </remarks>
-    public IList<McpRequestFilter<ListPromptsRequestParams, ListPromptsResult>> ListPromptsFilters { get; } = [];
+    public IList<McpRequestFilter<ListPromptsRequestParams, ListPromptsResult>> ListPromptsFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the get-prompt handler pipeline.
+    /// Gets or sets the filters for the get-prompt handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that are invoked when a client requests details for a specific prompt that isn't found in the <see cref="McpServerPrompt"/> collection.
     /// The filters can modify, log, or perform additional operations on requests and responses for
     /// <see cref="RequestMethods.PromptsGet"/> requests. The handler should implement logic to fetch or generate the requested prompt and return appropriate results.
     /// </remarks>
-    public IList<McpRequestFilter<GetPromptRequestParams, GetPromptResult>> GetPromptFilters { get; } = [];
+    public IList<McpRequestFilter<GetPromptRequestParams, GetPromptResult>> GetPromptFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the list-resource-templates handler pipeline.
+    /// Gets or sets the filters for the list-resource-templates handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that return a list of available resource templates when requested by a client.
@@ -70,10 +102,18 @@ public sealed class McpRequestFilters
     /// <see cref="RequestMethods.ResourcesTemplatesList"/> requests. It supports pagination through the cursor mechanism,
     /// where the client can make repeated calls with the cursor returned by the previous call to retrieve more resource templates.
     /// </remarks>
-    public IList<McpRequestFilter<ListResourceTemplatesRequestParams, ListResourceTemplatesResult>> ListResourceTemplatesFilters { get; } = [];
+    public IList<McpRequestFilter<ListResourceTemplatesRequestParams, ListResourceTemplatesResult>> ListResourceTemplatesFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the list-resources handler pipeline.
+    /// Gets or sets the filters for the list-resources handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that return a list of available resources when requested by a client.
@@ -81,20 +121,36 @@ public sealed class McpRequestFilters
     /// <see cref="RequestMethods.ResourcesList"/> requests. It supports pagination through the cursor mechanism,
     /// where the client can make repeated calls with the cursor returned by the previous call to retrieve more resources.
     /// </remarks>
-    public IList<McpRequestFilter<ListResourcesRequestParams, ListResourcesResult>> ListResourcesFilters { get; } = [];
+    public IList<McpRequestFilter<ListResourcesRequestParams, ListResourcesResult>> ListResourcesFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the read-resource handler pipeline.
+    /// Gets or sets the filters for the read-resource handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that are invoked when a client requests the content of a specific resource identified by its URI.
     /// The filters can modify, log, or perform additional operations on requests and responses for
     /// <see cref="RequestMethods.ResourcesRead"/> requests. The handler should implement logic to locate and retrieve the requested resource.
     /// </remarks>
-    public IList<McpRequestFilter<ReadResourceRequestParams, ReadResourceResult>> ReadResourceFilters { get; } = [];
+    public IList<McpRequestFilter<ReadResourceRequestParams, ReadResourceResult>> ReadResourceFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the complete-handler pipeline.
+    /// Gets or sets the filters for the complete-handler pipeline.
     /// </summary>
     /// <remarks>
     /// These filters wrap handlers that provide auto-completion suggestions for prompt arguments or resource references in the Model Context Protocol.
@@ -102,10 +158,18 @@ public sealed class McpRequestFilters
     /// <see cref="RequestMethods.CompletionComplete"/> requests. The handler processes auto-completion requests, returning a list of suggestions based on the
     /// reference type and current argument value.
     /// </remarks>
-    public IList<McpRequestFilter<CompleteRequestParams, CompleteResult>> CompleteFilters { get; } = [];
+    public IList<McpRequestFilter<CompleteRequestParams, CompleteResult>> CompleteFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the subscribe-to-resources handler pipeline.
+    /// Gets or sets the filters for the subscribe-to-resources handler pipeline.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -119,10 +183,18 @@ public sealed class McpRequestFilters
     /// whenever a relevant resource is created, updated, or deleted.
     /// </para>
     /// </remarks>
-    public IList<McpRequestFilter<SubscribeRequestParams, EmptyResult>> SubscribeToResourcesFilters { get; } = [];
+    public IList<McpRequestFilter<SubscribeRequestParams, EmptyResult>> SubscribeToResourcesFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the unsubscribe-from-resources handler pipeline.
+    /// Gets or sets the filters for the unsubscribe-from-resources handler pipeline.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -136,10 +208,18 @@ public sealed class McpRequestFilters
     /// to the client for the specified resources.
     /// </para>
     /// </remarks>
-    public IList<McpRequestFilter<UnsubscribeRequestParams, EmptyResult>> UnsubscribeFromResourcesFilters { get; } = [];
+    public IList<McpRequestFilter<UnsubscribeRequestParams, EmptyResult>> UnsubscribeFromResourcesFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for the set-logging-level handler pipeline.
+    /// Gets or sets the filters for the set-logging-level handler pipeline.
     /// </summary>
     /// <remarks>
     /// <para>
@@ -153,5 +233,13 @@ public sealed class McpRequestFilters
     /// at or above the specified level to the client as notifications/message notifications.
     /// </para>
     /// </remarks>
-    public IList<McpRequestFilter<SetLevelRequestParams, EmptyResult>> SetLoggingLevelFilters { get; } = [];
+    public IList<McpRequestFilter<SetLevelRequestParams, EmptyResult>> SetLoggingLevelFilters
+    {
+        get => field ??= [];
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 }

src/ModelContextProtocol.Core/Server/McpServerFilters.cs

@@ -12,12 +12,28 @@ namespace ModelContextProtocol.Server;
 public sealed class McpServerFilters
 {
     /// <summary>
-    /// Gets the filters for incoming and outgoing JSON-RPC messages.
+    /// Gets or sets the filters for incoming and outgoing JSON-RPC messages.
     /// </summary>
-    public McpMessageFilters Message { get; } = new();
+    public McpMessageFilters Message
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filters for request-specific MCP handler pipelines.
+    /// Gets or sets the filters for request-specific MCP handler pipelines.
     /// </summary>
-    public McpRequestFilters Request { get; } = new();
+    public McpRequestFilters Request
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 }

src/ModelContextProtocol.Core/Server/McpServerOptions.cs

@@ -97,17 +97,33 @@ public sealed class McpServerOptions
     /// <summary>
     /// Gets or sets the container of handlers used by the server for processing protocol messages.
     /// </summary>
-    public McpServerHandlers Handlers { get; } = new();
+    public McpServerHandlers Handlers
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
-    /// Gets the filter collections for MCP server handlers.
+    /// Gets or sets the filter collections for MCP server handlers.
     /// </summary>
     /// <remarks>
     /// This property provides access to filter collections that can be used to modify the behavior
     /// of various MCP server handlers. The first filter added is the outermost (first to execute),
     /// and each subsequent filter wraps closer to the handler.
     /// </remarks>
-    public McpServerFilters Filters { get; } = new();
+    public McpServerFilters Filters
+    {
+        get => field ??= new();
+        set
+        {
+            Throw.IfNull(value);
+            field = value;
+        }
+    }
 
     /// <summary>
     /// Gets or sets a collection of tools served by the server.

tests/ModelContextProtocol.TestSseServer/Program.cs

@@ -90,7 +90,9 @@ public class Program
         }
 
         const int pageSize = 10;
-        options.Handlers.ListToolsHandler = async (request, cancellationToken) =>
+        options.Handlers = new()
+        {
+            ListToolsHandler = async (request, cancellationToken) =>
             {
                 return new ListToolsResult
                 {
@@ -167,9 +169,9 @@ public class Program
                         }
                     ]
                 };
-        };
+            },
 
-        options.Handlers.CallToolHandler = async (request, cancellationToken) =>
+            CallToolHandler = async (request, cancellationToken) =>
             {
                 if (request.Params is null)
                 {
@@ -227,9 +229,9 @@ public class Program
                 {
                     throw new McpProtocolException($"Unknown tool: '{request.Params.Name}'", McpErrorCode.InvalidParams);
                 }
-        };
+            },
 
-        options.Handlers.ListResourceTemplatesHandler = async (request, cancellationToken) =>
+            ListResourceTemplatesHandler = async (request, cancellationToken) =>
             {
                 return new ListResourceTemplatesResult
                 {
@@ -241,9 +243,9 @@ public class Program
                     }
                     ]
                 };
-        };
+            },
 
-        options.Handlers.ListResourcesHandler = async (request, cancellationToken) =>
+            ListResourcesHandler = async (request, cancellationToken) =>
             {
                 int startIndex = 0;
                 var requestParams = request.Params ?? new();
@@ -274,9 +276,9 @@ public class Program
                     NextCursor = nextCursor,
                     Resources = resources.GetRange(startIndex, endIndex - startIndex)
                 };
-        };
+            },
 
-        options.Handlers.ReadResourceHandler = async (request, cancellationToken) =>
+            ReadResourceHandler = async (request, cancellationToken) =>
             {
                 if (request.Params?.Uri is null)
                 {
@@ -311,9 +313,9 @@ public class Program
                 {
                     Contents = [contents]
                 };
-        };
+            },
 
-        options.Handlers.ListPromptsHandler = async (request, cancellationToken) =>
+            ListPromptsHandler = async (request, cancellationToken) =>
             {
                 return new ListPromptsResult
                 {
@@ -345,9 +347,9 @@ public class Program
                         }
                     ]
                 };
-        };
+            },
 
-        options.Handlers.GetPromptHandler = async (request, cancellationToken) =>
+            GetPromptHandler = async (request, cancellationToken) =>
             {
                 if (request.Params is null)
                 {
@@ -397,6 +399,7 @@ public class Program
                 {
                     Messages = messages
                 };
+            },
         };
     }