Experience talk after socket programming project

Our project is to use dotnet to implement socket connection, with multithreading to deal with multiple users. I'll just talk about the problems I encountered during the implementation process. Here is a simplified implementation of my server.
Private Socket Clientsocket;
public void Start ()
{
Thread Thread=new Thread (new ThreadStart (Mainservice));
Thread. Start ();
}
void Mainservice ()
{
Listener=new TcpListener (111111);
Listener. Start ();
while (true)
{
Socket s= Listener. AcceptSocket ()//frame listening to customer connection
Clientsocket=s;
Thread clientservice = new Thread (new ThreadStart (ClientService));
ClientService. Start ();
Thread.Sleep (200);//1, wait to pass the value to the variable in the thread, prevent multiple users from causing conflict
}
}
void ClientService ()
{
string command;
String Receivestr;
String filename,filesize;
Socket Sock=clientsocket;
EndPoint Oldep=sock. Remoteendpoint;
Byte[] Bb=new byte[200];
while (true)
{
Ii=sock. ReceiveFrom (Bb,ref OLDEP);//Receive Data
if (ii==0)//2, client connection is disconnected, it will receive 0 bytes continuously
{
Break
}
Receivestr=encoding.default.getstring (BB,0,BB. Length);
Command=readlinefrom (receivestr,1). ToLower ();//readlinefrom read one row of data in a string
Switch (command)
{
Cace "Upload":
Filename=readlinefrom (receivestr,2);
Filesize=readlinefrom (receivestr,3);
UploadFile (Sock,filename,int32.parse (fileSize));//Accept data method,
Break
....................
}
}
void UploadFile (Socket sock,string filename,int fileSize)
{
int loadsize=0;
Byte[] BB;