I am making a simple tcp/ip chat program for practicing threads and tcp/ip. I was using asynchronous methods but had a problem with concurrency so I went to threads and blocking methods (not asynchronous). I have two private variables defined in the class, not static:

string amessage = string.Empty;
int MessageLength;

and a Thread

private Thread BeginRead;

Ok so I call a function called Listen ONCE when the client starts:

public virtual void Listen(int byteLength)
        {
            var state = new StateObject {Buffer = new byte[byteLength]};
            BeginRead = new Thread(ReadThread);
            BeginRead.Start(state);                
        }

and finally the function to receive commands and process them, I'm going to shorten it because it is really long:

  private void ReadThread(object objectState)
    {
       var state = (StateObject)objectState;
       int byteLength = state.Buffer.Length;
       while (true)
          {

              var buffer = new byte[byteLength];
              int len = MySocket.Receive(buffer);
              if (len <= 0) return;               
              string content = Encoding.ASCII.GetString(buffer, 0, len);
              amessage += cleanMessage.Substring(0, MessageLength);
              if (OnRead != null)
                {
                   var e = new CommandEventArgs(amessage);
                   OnRead(this, e);
                 }

           }

    }

Now, as I understand it only one thread at a time will enter BeginRead, I call Receive, it blocks until I get data, and then I process it. The problem: the variable amessage will change it's value between statements that do not touch or alter the variable at all, for example at the bottom of the function at: if (OnRead != null) "amessage" will be equal to 'asdf' and at if (OnRead != null) "amessage" will be equal to qwert. As I understand it this is indicative of another thread changing the value/running asynchronously. I only spawn one thread to do the receiving and the Receive function is blocking, how could there be two threads in this function and if there is only one thread how does amessage's value change between statements that don't affect it's value. As a side note sorry for spamming the site with these questions but I'm just getting a hang of this threading story and it's making me want to sip cyanide.

Thanks in advance.

EDIT:

Here is my code that calls the Listen Method in the client:

public void ConnectClient(string ip,int port)
        {

            client.Connect(ip,port);
            client.Listen(5);
        }

and in the server:

private void Accept(IAsyncResult result)
        {

                var client = new AbstractClient(MySocket.EndAccept(result));
                var e = new CommandEventArgs(client, null);
                    Clients.Add(client);
                    client.Listen(5);
                    if (OnClientAdded != null)
                    {
                        var target = (Control) OnClientAdded.Target;
                        if (target != null && target.InvokeRequired)
                            target.Invoke(OnClientAdded, this, e);
                        else
                            OnClientAdded(this, e);


                    }


                    client.OnRead += OnRead;
                    MySocket.BeginAccept(new AsyncCallback(Accept), null);

        }

All this code is in a class called AbstractClient. The client inherits the Abstract client and when the server accepts a socket it create's it's own local AbstractClient, in this case both modules access the functions above however they are different instances and I couldn't imagine threads from different instances combining especially as no variable is static.