private static ScreenBody screenBody=null;

private ScreenBody()
{
InitializeComponent();
}

private static ScreenBody GetSingle()
{
if(screenBody==null)
{
screenBody=new ScreenBody();
}
return screenBody;
}

private void btnCutter_Click(object sender,EventArgs e)
{
 //新建一個和屏幕大小相同的圖片img 也可以用BitMap
image img=new Bitmap(Screen.AllScreens[0].Bounds.Width,Screen.AllScreens[0].Bounds.Height);
//創(chuàng)建一個畫板 讓我們可以在畫板上畫圖 大小和屏幕大小一樣大
Graphics g=Graphics.FromImage(img);
 //將屏幕圖片拷貝到空白圖片img
g.CopyFromScreen(new Point(0,0),new Point(0,0),Screen.AllScreens[0].Bounds.Size);
//創(chuàng)建截圖窗體
ScreenBody body=ScreenBody.GetSingle();
//指示窗體的背景圖片為屏幕圖片
body.BackGroundImage=img;
body.ShowDialog();

}

private bool CatchStart;//判斷鼠標是否按下
private bool CatchFinished;//判斷矩形是否繪制完成
private Point DownPoint;//鼠標按下的點
private Image baseMap;//最基本的圖片
private Rectangle CatchRectangle;  

 private void ScreenBody_MouseDown(object sender, MouseEventArgs e)
  {
   //鼠標左鍵按下就是開始畫圖，也就是截圖
   if (e.Button == MouseButtons.Left)
   {
    if (CatchStart == false)
    {
     CatchStart = true;
     //保存此時的坐標
     DownPoint = new Point(e.X, e.Y);
    }
   }
  }

 private void ScreenBody_MouseMove(object sender, MouseEventArgs e)
  {
   //確保截圖開始
   if (CatchStart)
   {
    //新建一個圖片，讓它與屏幕圖片相同
    Bitmap copyBmp = (Bitmap)baseMap.Clone();
    //鼠標按下時的坐標
    Point newPoint = new Point(DownPoint.X, DownPoint.Y);

    //新建畫板和畫筆
    Graphics g = Graphics.FromImage(copyBmp);
    Pen p = new Pen(Color.Azure, 1);//畫筆的顏色為azure 寬度為1

    //獲取矩形的長度 
    int width = Math.Abs(e.X - DownPoint.Y);
    int height = Math.Abs(e.Y - DownPoint.Y);

    if (e.X < DownPoint.X)
    {
     newPoint.X = e.X;

    }
    if (e.Y < DownPoint.Y)
    {
     newPoint.Y = e.Y;
    }

    CatchRectangle = new Rectangle(newPoint, new Size(width, height));
    g.DrawRectangle(p, CatchRectangle);

    //釋放目前的畫板
    g.Dispose();
    p.Dispose();

    //從當前窗體創(chuàng)建新的畫板
    Graphics g1 = this.CreateGraphics();
    //將剛剛所畫的圖片畫到截圖窗體上去
    //為什么不直接在當前窗體畫圖呢？？？
    //如果直接解決將矩形畫在窗體上，會造成圖片抖動而且有多個矩形
    //這樣實現(xiàn)也屬于二次緩沖技術
    g1.DrawImage(copyBmp, new Point(0, 0));
    g1.Dispose();

    //釋放拷貝圖片 防止內(nèi)存被大量的消耗
    copyBmp.Dispose();
   }

 /// <summary>
  /// 鼠標左鍵彈起事件
  /// </summary>
  /// <param name="sender"></param>
  /// <param name="e"></param>
  private void ScreenBody_MouseUp(object sender, MouseEventArgs e)
  {
   if (e.Button == MouseButtons.Left)
   {
    //如果截圖已經(jīng)開始，鼠標左鍵彈起設置截圖完成
    if (CatchStart)
    {
     CatchStart = false;
     CatchFinished = true;
    }
   }
  }

 private void ScreenBody_MouseDoubleClick(object sender, MouseEventArgs e)
  {
   if (e.Button==MouseButtons.Left&&CatchFinished)
   {
    //新建一個矩形大小相同的空白圖片
    Bitmap CatcheBmp = new Bitmap(CatchRectangle.Width, CatchRectangle.Height);
    Graphics g = Graphics.FromImage(CatcheBmp); ;
   
    //把basemap中指定的部分按照指定大小畫到空白圖片上
    //CatchRectangle指定的baseMap中指定的部分
    //第二個參數(shù)指定繪制到空白圖片的位置和大小
    //畫完后CatchedBmp不再是空白圖片，而是具有與截取的圖片一樣的內(nèi)容
    g.DrawImage(baseMap, new Rectangle(0, 0, CatchRectangle.Width, CatchRectangle.Height));

    //將圖片保存到剪切板中
    Clipboard.SetImage(CatcheBmp);
    g.Dispose();

    CatchFinished = false;
    this.BackgroundImage = baseMap;
    CatcheBmp.Dispose();
    this.DialogResult = DialogResult.OK;
    this.Close();
   }
  }

/// <summary>
  /// 鼠標右鍵點擊結(jié)束截圖
  /// </summary>
  /// <param name="sender"></param>
  /// <param name="e"></param>
  private void ScreenBody_MouseClick(object sender, MouseEventArgs e)
  {
   if (e.Button == MouseButtons.Right)
   {
    this.DialogResult = DialogResult.OK;
    this.Close();
   }
  }

[FlagsAttribute]
 public enum KeyModifiers
 {
  None = 0,
  Alt = 1,
  Ctrl = 2,
  Shift = 4,
  WindowsKey = 8
 }

 //在C#中引用命名空間System.Runtime.InteropServices;來加載非托管類user32.dll
  /*
  * RegisterHotKey函數(shù)原型及說明：
  * BOOL RegisterHotKey(
  * HWND hWnd,   // window to receive hot-key notification
  * int id,   // identifier of hot key
  * UINT fsModifiers, // key-modifier flags
  * UINT vk   // virtual-key code);
  * 參數(shù) id為你自己定義的一個ID值
  * 對一個線程來講其值必需在0x0000 - 0xBFFF范圍之內(nèi),十進制為0~49151
  * 對DLL來講其值必需在0xC000 - 0xFFFF 范圍之內(nèi),十進制為49152~65535
  * 在同一進程內(nèi)該值必須唯一參數(shù) fsModifiers指明與熱鍵聯(lián)合使用按鍵
  * 可取值為：MOD_ALT MOD_CONTROL MOD_WIN MOD_SHIFT參數(shù)，或數(shù)字0為無，1為Alt,2為Control，4為Shift，8為Windows
  * vk指明熱鍵的虛擬鍵碼
  */
  [System.Runtime.InteropServices.DllImport("user32.dll")] //申明API函數(shù)
  public static extern bool RegisterHotKey(
   IntPtr hWnd, // handle to window
   int id, // hot key identifier
   uint fsModifiers, // key-modifier options
   Keys vk // virtual-key code
  );

  [System.Runtime.InteropServices.DllImport("user32.dll")] //申明API函數(shù)
  public static extern bool UnregisterHotKey(
   IntPtr hWnd, // handle to window
   int id // hot key identifier
  );

 private void Form1_Load(object sender, EventArgs e)
  {
   uint ctrlHotKey = (uint)(KeyModifiers.Alt | KeyModifiers.Ctrl);
   // 注冊熱鍵為Alt+Ctrl+C, "100"為唯一標識熱鍵
   RegisterHotKey(Handle, 100, ctrlHotKey, Keys.A);
  }
  //熱鍵按下執(zhí)行的方法
  private void GlobalKeyProcess()
  {
   this.WindowState = FormWindowState.Minimized;
   //窗口最小化需要一定的時間 使用線程
   Thread.Sleep(200);
   btnCutter.PerformClick();
  }

  protected override void WndProc(ref Message m)
  {
   //如果m.Msg的值為0x0312那么表示用戶按下了熱鍵
   const int WM_HOTKEY = 0x0312;
   switch (m.Msg)
   {
    case WM_HOTKEY:
     if (m.WParam.ToString()=="100")
     {
      GlobalKeyProcess();
     }
     break;
    default:
     break;
   }
   base.WndProc(ref m);
  }

  private void Form1_FormClosing(object sender, FormClosingEventArgs e)
  {
   // 卸載熱鍵
   UnregisterHotKey(Handle, 100);
  }