一个网络摄像头的forge

我用两个按钮和一个图片框编写了一个访问摄像头的代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using AForge.Imaging;
using AForge.Imaging.Filters;
using AForge.Video;
using AForge.Video.DirectShow;
namespace cam
{
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private FilterInfoCollection webcam;
    private VideoCaptureDevice cam;
    Bitmap bitmap;
    private void Form1_Load(object sender, EventArgs e)
    {
        webcam = new FilterInfoCollection(FilterCategory.VideoInputDevice);
        foreach (FilterInfo VideoCaptureDevice in webcam)
        {
            comboBox1.Items.Add(VideoCaptureDevice.Name);
        }
        comboBox1.SelectedIndex = 0;
      }
    private void button1_Click(object sender, EventArgs e)
    {
        cam = new VideoCaptureDevice(webcam[comboBox1.SelectedIndex].MonikerString);
        cam.NewFrame += new NewFrameEventHandler(cam_NewFrame);
        cam.Start();
    }
    void cam_NewFrame(object sender, NewFrameEventArgs eventArgs)
    {
        Bitmap bitmap = (Bitmap)eventArgs.Frame.Clone();
        pictureBox1.Image = bitmap;
    }

    private void button3_Click(object sender, EventArgs e)
    {
        if (cam.IsRunning)
        {
            cam.Stop();
        }
    }
    private void pictureBox1_Click(object sender, EventArgs e)
    {
        pictureBox1.Image = bitmap;
    }

    }
 }

代码构建成功。但在调试图片盒不工作。启停工作正常。有人能帮忙吗?