C# Re-implementation (#5)

Re-implementation of C# interceptors SDK with clearer focus on
server-side / gateway interceptors.
Fixes a lot of issues with the existing implementation.
Still more boilerplate than I'm happy with, but this is better than what
was there. Will refine further in later PRs.

csharp/sdk/samples/TransparentProxySample/Program.cs

@@ -0,0 +1,73 @@
+using System.Runtime.CompilerServices;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Interceptors.Gateway;
+using ModelContextProtocol.Interceptors.Protocol;
+using ModelContextProtocol.Server;
+
+// ──────────────────────────────────────────────────────────────────────
+// Transparent Proxy Sample
+//
+// Creates a transparent MCP proxy that:
+//   1. Connects as a CLIENT to a backend MCP server (everything server)
+//   2. Connects as a CLIENT to an interceptor server (PII + email interceptors)
+//   3. Exposes itself as a SERVER via stdio
+//
+// To connecting clients (e.g. Claude Desktop), this proxy appears to be
+// the everything server — but all requests flow through the interceptor
+// chain first (validation, mutation, sink).
+//
+// Claude Desktop config example:
+//   {
+//     "mcpServers": {
+//       "everything-with-interceptors": {
+//         "command": "dotnet",
+//         "args": ["run", "--project", "path/to/TransparentProxySample"]
+//       }
+//     }
+//   }
+// ──────────────────────────────────────────────────────────────────────
+
+// 1. Connect to the interceptor server
+var interceptorServerPath = Path.Combine(GetSourceDir(), "..", "InterceptorServerSample");
+await using var interceptorClient = await McpClient.CreateAsync(
+    new StdioClientTransport(new()
+    {
+        Name = "InterceptorServer",
+        Command = "dotnet",
+        Arguments = ["run", "--project", interceptorServerPath],
+    }));
+
+// 2. Connect to the backend everything server
+await using var backendClient = await McpClient.CreateAsync(
+    new StdioClientTransport(new()
+    {
+        Name = "EverythingServer",
+        Command = "npx",
+        Arguments = ["-y", "@modelcontextprotocol/server-everything"],
+    }));
+
+// 3. Create the gateway
+await using var gateway = new McpInterceptorGateway(new McpInterceptorGatewayOptions
+{
+    BackendClient = backendClient,
+    InterceptorClients = [interceptorClient],
+    Events = [InterceptionEvents.ToolsCall], // Only intercept tools/call
+});
+
+// 4. Configure server options and create the proxy server on stdio
+var serverOptions = new McpServerOptions
+{
+    ServerInfo = new() { Name = "interceptor-proxy", Version = "1.0.0" },
+};
+gateway.ConfigureServerOptions(serverOptions);
+
+await using var server = McpServer.Create(new StdioServerTransport("interceptor-proxy"), serverOptions);
+
+// 5. Forward backend notifications to connected clients
+gateway.RegisterNotificationForwarding(server);
+
+// 6. Run the proxy — it will serve requests until the client disconnects
+await server.RunAsync();
+
+static string GetSourceDir([CallerFilePath] string? path = null) =>
+    Path.GetDirectoryName(path) ?? throw new InvalidOperationException();

csharp/sdk/samples/TransparentProxySample/TransparentProxySample.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="ModelContextProtocol" Version="1.1.0" />
+    <ProjectReference Include="..\..\src\ModelContextProtocol.Interceptors\ModelContextProtocol.Interceptors.csproj" />
+  </ItemGroup>
+
+</Project>