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/GatewaySample/GatewaySample.csproj

@@ -0,0 +1,16 @@
+<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>

csharp/sdk/samples/GatewaySample/Program.cs

@@ -0,0 +1,133 @@
+using System.Runtime.CompilerServices;
+using ModelContextProtocol.Client;
+using ModelContextProtocol.Interceptors.Client;
+using ModelContextProtocol.Interceptors.Protocol;
+
+// ──────────────────────────────────────────────────────────────────────
+// Gateway Sample
+//
+// Demonstrates a full gateway chain:
+//   Client ──▶ Interceptor Server (PII validator + email redactor) ──▶ Everything Server
+//
+// The interceptor server hosts three interceptors:
+//   1. pii-validator    (validation)  – blocks payloads containing SSN-like data
+//   2. email-redactor   (mutation)    – replaces email addresses with [EMAIL_REDACTED]
+//   3. request-logger   (sink) – logs all events to stderr
+// ──────────────────────────────────────────────────────────────────────
+
+Console.WriteLine("=== MCP Interceptors Gateway Sample ===");
+Console.WriteLine();
+
+// 1. Connect to the interceptor server (our InterceptorServerSample, launched via dotnet run)
+Console.WriteLine("[setup] Starting 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 everything server (MCP reference server via npx)
+Console.WriteLine("[setup] Starting everything server...");
+await using var everythingClient = await McpClient.CreateAsync(
+    new StdioClientTransport(new()
+    {
+        Name = "EverythingServer",
+        Command = "npx",
+        Arguments = ["-y", "@modelcontextprotocol/server-everything"],
+    }));
+
+// 3. Create the gateway wrapper
+var gateway = new InterceptingMcpClient(everythingClient, new InterceptingMcpClientOptions
+{
+    InterceptorClient = interceptorClient,
+    Events = [InterceptionEvents.ToolsCall],
+});
+
+// 4. List available interceptors
+Console.WriteLine("[setup] Connected! Listing interceptors...");
+var interceptors = await gateway.ListInterceptorsAsync();
+Console.WriteLine();
+foreach (var i in interceptors.Interceptors)
+{
+    var events = string.Join(", ", i.Hooks.SelectMany(h => h.Events).Distinct());
+    Console.WriteLine($"  interceptor: {i.Name,-20} type={i.Type,-15} events=[{events}]");
+}
+
+// 5. List tools from the everything server (passes through gateway)
+Console.WriteLine();
+var tools = await gateway.ListToolsAsync();
+Console.WriteLine($"[tools] {tools.Count} tools available: {string.Join(", ", tools.Select(t => t.Name))}");
+
+// ── Demo 1: Normal tool call (passes through cleanly) ──────────────
+Console.WriteLine();
+Console.WriteLine("── Demo 1: Normal echo (should pass through) ──");
+try
+{
+    var result = await gateway.CallToolAsync("echo", new Dictionary<string, object?>
+    {
+        ["message"] = "Hello from the gateway!"
+    });
+    Console.WriteLine($"  Result: {result.Content.FirstOrDefault()}");
+}
+catch (McpInterceptorValidationException ex)
+{
+    Console.WriteLine($"  BLOCKED: {ex.Message}");
+}
+
+// ── Demo 2: Tool call with email (mutation - email gets redacted) ──
+Console.WriteLine();
+Console.WriteLine("── Demo 2: Echo with email (should be redacted by email-redactor) ──");
+try
+{
+    var result = await gateway.CallToolAsync("echo", new Dictionary<string, object?>
+    {
+        ["message"] = "Contact me at john.doe@example.com for details"
+    });
+    Console.WriteLine($"  Result: {result.Content.FirstOrDefault()}");
+}
+catch (McpInterceptorValidationException ex)
+{
+    Console.WriteLine($"  BLOCKED: {ex.Message}");
+}
+
+// ── Demo 3: Tool call with PII (validation - gets blocked) ─────────
+Console.WriteLine();
+Console.WriteLine("── Demo 3: Echo with SSN reference (should be blocked by pii-validator) ──");
+try
+{
+    var result = await gateway.CallToolAsync("echo", new Dictionary<string, object?>
+    {
+        ["message"] = "My SSN is 123-45-6789"
+    });
+    Console.WriteLine($"  Result: {result.Content.FirstOrDefault()}");
+}
+catch (McpInterceptorValidationException ex)
+{
+    Console.WriteLine($"  BLOCKED: {ex.Message}");
+}
+
+// ── Demo 4: Normal add (should pass through) ───────────────────────
+Console.WriteLine();
+Console.WriteLine("── Demo 4: Add 17 + 25 (should pass through) ──");
+try
+{
+    var result = await gateway.CallToolAsync("get-sum", new Dictionary<string, object?>
+    {
+        ["a"] = 17,
+        ["b"] = 25,
+    });
+    Console.WriteLine($"  Result: {result.Content.FirstOrDefault()}");
+}
+catch (McpInterceptorValidationException ex)
+{
+    Console.WriteLine($"  BLOCKED: {ex.Message}");
+}
+
+Console.WriteLine();
+Console.WriteLine("=== Done ===");
+
+static string GetSourceDir([CallerFilePath] string? path = null) =>
+    Path.GetDirectoryName(path) ?? throw new InvalidOperationException();