c#线程间共享的数据不会更新

我要做的是我在gui线程中绘制图像，我有另一个线程，它对具有图像的对象进行了理想化，并每秒将其发送到tcp套接字。paint事件中的图像正在改变，但它在tcp线程中没有改变。下面是修改后的代码:

// GUI class:
private static readonly object lock_obj = new object();
private GraphicsServerReceiver server;
private Bitmap gl_image;
private void checkBoxActivateBroadcast_CheckedChanged(object sender, EventArgs e)
{
    if (checkBoxActivateBroadcast.Checked)
    {
        this.server = new GraphicsServerReceiver(SERVER_PORT, gl_image);
    }
}
private void glControl1_Paint(object sender, PaintEventArgs e)
{
    // do some drawing
    lock (lock_obj)
    {
       gl_image = TakeScreenshot();
    }
}
public Bitmap TakeScreenshot()
{
    if (GraphicsContext.CurrentContext == null)
            throw new GraphicsContextMissingException();
    int w = glControl1.ClientSize.Width;
    int h = glControl1.ClientSize.Height;
    Bitmap bmp = new Bitmap(w, h);
    System.Drawing.Imaging.BitmapData data =
        bmp.LockBits(glControl1.ClientRectangle, System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
    GL.ReadPixels(0, 0, glControl1.ClientSize.Width, glControl1.ClientSize.Height, PixelFormat.Bgr, PixelType.UnsignedByte, data.Scan0);
    bmp.UnlockBits(data);
    bmp.RotateFlip(RotateFlipType.RotateNoneFlipY);
    return bmp;
}
#----------------
# the GraphicsServerReceiver class:
private Bitmap gl_image;
private static readonly object lock_obj = new object();
public GraphicsServerReceiver(int port, Bitmap gl_image)
{
    this.port = port;
    this.gl_image = gl_image;
    this.tcpListener = new TcpListener(IPAddress.Loopback, port);
    this.tcpListener.Start();
    this.listenThread = new Thread(new ThreadStart(ListenForClients));
    this.listenThread.Start();
}
private void ListenForClients()
{
    while (true)
    {
        try
        {
            TcpClient client = this.tcpListener.AcceptTcpClient();
            Thread clientThread = new Thread(new ParameterizedThreadStart(HandleClientComm));
            clientThread.Start(client);
        }
        catch (Exception ex)
        {
            Console.WriteLine("Error: Server was stopped so program failed to listen to clients. " + ex.Message);
        }
    }
}
private void HandleClientComm(object client)
{
    byte[] buffer;
    lock(lock_obj)
    {
        databag.img = gl_image;
        buffer = Serializer.ObjectToByteArray(databag);
    }
    // send the data in buffer...
}

这一定是一个多线程问题，但我没有看到它。我在这两个类中使用了不同的锁对象。会是这个原因吗?

新增信息:下面是一个文件中的完整代码，它再现了没有paint事件的问题:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private static readonly object lock_obj = new object();
        private Bitmap gl_image;
        private GraphicsServerReceiver server;
        private bool server_running = false;
        public Form1()
        {
            InitializeComponent();
        }
        private void btn1_Click(object sender, EventArgs e)
        {
            if (!server_running)
            {
                lock (lock_obj)
                {
                    gl_image = new Bitmap("C:/temp/x.bmp");
                }
                this.server = new GraphicsServerReceiver(gl_image);
                btn1.Text = "stop";
            }
            else
            {
                server.stop();
                btn1.Text = "start";
            }
            server_running = !server_running;
        }
        private void btn2_Click(object sender, EventArgs e)
        { // change the image
            lock (lock_obj)
            {
                // draw a line:
                using (var graphics = Graphics.FromImage(gl_image))
                {
                    Pen blackPen = new Pen(Color.Black, 3);
                    graphics.DrawLine(blackPen, 0, 0, 50, 50);
                }
                gl_image.Save("C:/temp/changed.bmp");
            }
        }
    }
    class GraphicsServerReceiver
    {
        private Bitmap gl_image;
        private static readonly object lock_obj = new object();
        private Thread listenThread;
        public void stop()
        {
            listenThread.Abort();
            listenThread.Join(1);
        }
        public GraphicsServerReceiver(Bitmap gl_image)
        {
            this.gl_image = gl_image;
            this.listenThread = new Thread(new ThreadStart(ListenForClients));
            this.listenThread.Start();
        }
        private void ListenForClients()
        {
            try
            {
                Thread clientThread = new Thread(new ParameterizedThreadStart(HandleClientComm));
                clientThread.Start("some param");
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error: " + ex.Message);
            }
        }
        private void HandleClientComm(object useless_obj)
        {
            while (listenThread.IsAlive)
            {
                lock (lock_obj)
                {
                    gl_image.Save("C:/temp/out.bmp");
                }
                Thread.Sleep(1000);
            }
        }
    }
}