C# WinForm：FormBorderStyle=None时如何实现窗体可缩放？

当把FormBorderStyle设为None隐藏标题栏后，默认会失去缩放功能，以下是两种实用实现方式：

方法一：重写WndProc拦截系统消息 #

通过拦截Windows系统消息WM_NCHITTEST，让窗体的边缘区域被系统识别为可缩放的边框，从而保留原生缩放体验。

在窗体类中添加以下代码：

protected override void WndProc(ref Message m)
{
    const int WM_NCHITTEST = 0x0084;
    const int HTLEFT = 10;
    const int HTRIGHT = 11;
    const int HTTOP = 12;
    const int HTTOPLEFT = 13;
    const int HTTOPRIGHT = 14;
    const int HTBOTTOM = 15;
    const int HTBOTTOMLEFT = 16;
    const int HTBOTTOMRIGHT = 17;

    base.WndProc(ref m);

    if (m.Msg == WM_NCHITTEST && (int)m.Result == 1)
    {
        Point pos = new Point(m.LParam.ToInt32() & 0xffff, m.LParam.ToInt32() >> 16);
        pos = this.PointToClient(pos);
        if (pos.X <= 5)
            if (pos.Y <= 5)
                m.Result = (IntPtr)HTTOPLEFT;
            else if (pos.Y >= ClientSize.Height - 5)
                m.Result = (IntPtr)HTBOTTOMLEFT;
            else
                m.Result = (IntPtr)HTLEFT;
        else if (pos.X >= ClientSize.Width - 5)
            if (pos.Y <= 5)
                m.Result = (IntPtr)HTTOPRIGHT;
            else if (pos.Y >= ClientSize.Height - 5)
                m.Result = (IntPtr)HTBOTTOMRIGHT;
            else
                m.Result = (IntPtr)HTRIGHT;
        else if (pos.Y <= 5)
            m.Result = (IntPtr)HTTOP;
        else if (pos.Y >= ClientSize.Height - 5)
            m.Result = (IntPtr)HTBOTTOM;
    }
}

说明：代码中5是可缩放区域的宽度，可根据需求调整；设置FormBorderStyle = None后，运行窗体就能通过边缘拖拽缩放。

方法二：手动处理鼠标事件 #

通过监听鼠标的按下、移动、释放事件，手动计算窗体的大小变化，适合需要更自定义缩放逻辑的场景。

步骤如下：

• 定义全局变量记录状态：

  private bool isResizing = false;
  private int resizeBorderWidth = 5;
  private enum ResizeDirection { None, Left, Right, Top, Bottom, TopLeft, TopRight, BottomLeft, BottomRight }
  private ResizeDirection currentResizeDir = ResizeDirection.None;
  

• 实现MouseDown事件：

  private void Form1_MouseDown(object sender, MouseEventArgs e)
  {
      currentResizeDir = GetResizeDirection(e.Location);
      isResizing = currentResizeDir != ResizeDirection.None;
      if (isResizing)
          Cursor = GetResizeCursor(currentResizeDir);
  }
  

• 实现MouseMove事件：

  private void Form1_MouseMove(object sender, MouseEventArgs e)
  {
      if (!isResizing)
      {
          var dir = GetResizeDirection(e.Location);
          Cursor = GetResizeCursor(dir);
          return;
      }
  
      switch (currentResizeDir)
      {
          case ResizeDirection.Right:
              Width = Math.Max(MinimumSize.Width, e.X);
              break;
          case ResizeDirection.Bottom:
              Height = Math.Max(MinimumSize.Height, e.Y);
              break;
          case ResizeDirection.BottomRight:
              Width = Math.Max(MinimumSize.Width, e.X);
              Height = Math.Max(MinimumSize.Height, e.Y);
              break;
          case ResizeDirection.Left:
              int newX = Left + e.X;
              int newWidth = Width - e.X;
              if (newWidth >= MinimumSize.Width)
              {
                  Left = newX;
                  Width = newWidth;
              }
              break;
          case ResizeDirection.Top:
              int newY = Top + e.Y;
              int newHeight = Height - e.Y;
              if (newHeight >= MinimumSize.Height)
              {
                  Top = newY;
                  Height = newHeight;
              }
              break;
          case ResizeDirection.TopLeft:
              newX = Left + e.X;
              newWidth = Width - e.X;
              newY = Top + e.Y;
              newHeight = Height - e.Y;
              if (newWidth >= MinimumSize.Width && newHeight >= MinimumSize.Height)
              {
                  Left = newX;
                  Width = newWidth;
                  Top = newY;
                  Height = newHeight;
              }
              break;
          case ResizeDirection.TopRight:
              newWidth = Math.Max(MinimumSize.Width, e.X);
              newY = Top + e.Y;
              newHeight = Height - e.Y;
              if (newHeight >= MinimumSize.Height)
              {
                  Top = newY;
                  Height = newHeight;
                  Width = newWidth;
              }
              break;
          case ResizeDirection.BottomLeft:
              newX = Left + e.X;
              newWidth = Width - e.X;
              newHeight = Math.Max(MinimumSize.Height, e.Y);
              if (newWidth >= MinimumSize.Width)
              {
                  Left = newX;
                  Width = newWidth;
                  Height = newHeight;
              }
              break;
      }
  }
  

• 实现MouseUp事件：

  private void Form1_MouseUp(object sender, MouseEventArgs e)
  {
      isResizing = false;
      Cursor = Cursors.Default;
      currentResizeDir = ResizeDirection.None;
  }
  

• 辅助方法：

  private ResizeDirection GetResizeDirection(Point pos)
  {
      bool isLeft = pos.X <= resizeBorderWidth;
      bool isRight = pos.X >= ClientSize.Width - resizeBorderWidth;
      bool isTop = pos.Y <= resizeBorderWidth;
      bool isBottom = pos.Y >= ClientSize.Height - resizeBorderWidth;
  
      if (isLeft && isTop) return ResizeDirection.TopLeft;
      if (isRight && isTop) return ResizeDirection.TopRight;
      if (isLeft && isBottom) return ResizeDirection.BottomLeft;
      if (isRight && isBottom) return ResizeDirection.BottomRight;
      if (isLeft) return ResizeDirection.Left;
      if (isRight) return ResizeDirection.Right;
      if (isTop) return ResizeDirection.Top;
      if (isBottom) return ResizeDirection.Bottom;
      return ResizeDirection.None;
  }
  
  private Cursor GetResizeCursor(ResizeDirection dir)
  {
      switch (dir)
      {
          case ResizeDirection.Left:
          case ResizeDirection.Right:
              return Cursors.SizeWE;
          case ResizeDirection.Top:
          case ResizeDirection.Bottom:
              return Cursors.SizeNS;
          case ResizeDirection.TopLeft:
          case ResizeDirection.BottomRight:
              return Cursors.SizeNWSE;
          case ResizeDirection.TopRight:
          case ResizeDirection.BottomLeft:
              return Cursors.SizeNESW;
          default:
              return Cursors.Default;
      }
  }
  

说明：同样需要设置FormBorderStyle = None，运行后拖拽窗体边缘即可缩放，还可自定义缩放区域宽度和光标样式。

内容的提问来源于stack exchange，提问作者Mahbub Alam