[EDIT] made main window open only when the client has connected

Client/Client.cs

@@ -17,6 +17,7 @@ namespace Client
         public int Port = 5555;
         public bool Connected = false;
         private string username;
+        public Callback OnSuccessfullConnect;
 
         public Client(string username)
         {
@@ -31,6 +32,7 @@ namespace Client
             Debug.Write("finished connecting to server");
             this.tcpClient.EndConnect(ar);
             this.stream = tcpClient.GetStream();
+            OnSuccessfullConnect?.Invoke();
             SendMessage(JSONConvert.ConstructUsernameMessage(username));
             this.stream.BeginRead(buffer, 0, buffer.Length, new AsyncCallback(OnReadComplete),null);
         }

Client/Views/LoginScreen.xaml.cs

@@ -28,13 +28,18 @@ namespace Client.Views
         {
             User user = new User(usernameTextbox.Text);
             Client client = new Client(user.Username);
+            client.OnSuccessfullConnect = () =>
+            {
+                // because we need to start the main window on a UI thread, we need to let the dispatcher handle it, which will execute the code on the ui thread
+                Application.Current.Dispatcher.Invoke(delegate {
+                    data.User = user;
+                    data.Client = client;
+                    MainWindow startWindow = new MainWindow();
+                    startWindow.Show();
+                    this.Close();
+                });
+            };
 
-            data.User = user;
-            data.Client = client;
-
-            MainWindow startWindow = new MainWindow();
-            startWindow.Show();
-            this.Close();
         }
     }
 }

SharedClientServer/ClientServerUtil.cs

@@ -6,6 +6,7 @@ using System.Text;
 
 namespace SharedClientServer
 {
+    public delegate void Callback();
     class ClientServerUtil
     {
         // creates a message array to send to the server or to clients