Unit testing async tcp server

Question

I built an async multi-client TCP server for RPC usage. It's working well but I've found it difficult to unit test certain functionality:

1. Connect 2x clients, is client count 2
2. Connect 1x client, disconnect client, is client count zero

I want to test that the server is robust with handling disconnects and multiple connections. The below test fails only due to scheduling.

Unit Test

 [TestMethod]
 public void Start_TwoConnections_ClientsIsTwo()
 {
 var handler = new HandlerStub();
 using (server = new APIServer(handler))
 using (var client1 = new TcpClient())
 using (var client2 = new TcpClient())
 {
 server.Start();
 client1.Connect(IPAddress.Loopback, port);
 client2.Connect(IPAddress.Loopback, port);
 // await Task.Delay(500); <-- This will fix the problem, but is surely unreliable.
 Assert.AreEqual(2, server.Clients);
 }
 }

Server Snippet

 public void Start()
 {
 // Root try-catch, for unexpected errors
 try
 {
 server = new TcpListener(IPAddress.Loopback, 8352);
 IsRunning = true;
 do // Retry loop
 {
 // Start server errors
 try
 {
 server.Start();
 var task = Task.Run(AcceptConnections);
 }
 catch (SocketException ex)
 {
 Console.WriteLine(string.Format("Error {0}: Failed to start server.", ex.ErrorCode));
 }
 }
 while (!server.Server.IsBound && !IsDisposed);
 }
 catch (Exception ex)
 {
 IsRunning = false;
 Console.WriteLine(string.Format("Unexpected Error: {0}", ex.ToString()));
 throw ex;
 }
 }
 private async Task AcceptConnections()
 {
 try
 { 
 // Multi-client listener loop
 do
 {
 var connection = await AcceptConnection();
 connections.Add(connection);
 }
 while (!IsDisposed && IsRunning);
 }
 catch (SocketException ex)
 {
 Console.WriteLine(string.Format("Error {0}: Server socket error.", ex.ErrorCode));
 CleanupConnections();
 }
 }

How can this code be refactored to improve it's test-ability?

Answer 1

By exposing TcpListener.Pending() on my APIServer, I was able to resolve this.

APIServer.cs

 TcpListener server;
 public bool Pending
 {
 get
 {
 return server.Pending();
 }
 }

Unit Test

 [TestMethod]
 public async Task Start_TwoConnections_ClientsIsTwo()
 {
 var gazeStub = new GazeServerStub();
 using (server = new APIServer(gazeStub))
 using (var client1 = new TcpClient())
 using (var client2 = new TcpClient())
 {
 server.Start();
 client1.Connect(IPAddress.Loopback, port);
 client2.Connect(IPAddress.Loopback, port);
 await WaitForNoPendingConnections(server);
 Assert.AreEqual(2, server.Clients);
 }
 }
 private async Task WaitForNoPendingConnections(APIServer server)
 {
 while (server.Pending)
 {
 await Task.Delay(50);
 }
 }