C# Upgrade + Bugs Fixes of an UDP Chat Server + Client

I am making a Chat Server + Client by using UDP Protocol This chat server is work like Game World Chat, when one player chat, all players in that game can see the chat. So basically, when one client send chat, then all clients that all connected can see that chat too. Well for some reason, i'm gonna need your help to upgrade and bugs fixes. Information: --------------- Script: C# + .NET Engine: Microsoft Visual Studio 2015 Framework: .NET Framework 4.6 OS: Windows 7 or above Your Job: ----------- 1. Bug fix especially when there is two or more clients connected to server, then one of the client close ungracefully, it cause error on server and closing the server application, how to fix this? 2. How to detect if the clients has no longer connected to the server on UDP Socket? Because so far i know, UDP is connectionless protocol, so it is almost impossible to detect which one client has been disconnected yet. 3. Please do upgrade my script to become more secure connection, so no one can't hijack / hack / crack it easily 4. Please do upgrade my script to become light / faster / smooth, because my hope this server capable to handle over 10.000 clients realtime running 24/7 Non-Stop 5. Your suggestion ... --> Well, i'm not perfect, so i want to hear any suggestion from you to upgrade my server, if you have an idea to improve my server besides the other 4 points above, i would like to hear it from you, and we are gonna talk some more about extra charge. The server and client is console based, so you don't need to give any UI/GUI on it. Just use simple command console like "/connect" "/shutdown" etc... BEWARE: ------------- Because i'm on a tight schedule, i'm must force you to finish this project between 3-5 days You can get the file of my server publicly at this project attachment... Please download and learn it before you put a bid on my project.