using System.Runtime.InteropServices;

public delegate IntPtr HookProc(int nCode, IntPtr wParam, IntPtr lParam);

[DllImport("user32.dll")]
public static extern IntPtr SetWindowsHookEx(int hookid, 
    HookProc pfnhook, IntPtr hinst, int threadid);

[DllImport("user32.dll")]
public static extern IntPtr CallNextHookEx(IntPtr hhook, 
    int code, IntPtr wparam, IntPtr lparam);

[DllImport("kernel32.dll")]
public static extern IntPtr GetModuleHandle(string modName);

[DllImport("user32.dll")]
public static extern bool UnhookWindowsHookEx(IntPtr hhook);

[DllImport("user32.dll")]
public static extern bool GetWindowRect(IntPtr hWnd, ref Rectangle rect);

[DllImport("user32.dll")]
public static extern bool MoveWindow(
    IntPtr hWnd, int X, int Y, int nWidth, int nHeight, bool bRepaint);

public const int WH_CBT = 5;
public const int HCBT_ACTIVATE = 5;
IntPtr hookHandle = IntPtr.Zero;

private IntPtr CBTHookCallback(int nCode, IntPtr wParam, IntPtr lParam)
{
     switch(nCode)
     {
         case HCBT_ACTIVATE:
             Rectangle vRectangle = new Rectangle();
             GetWindowRect(wParam, ref vRectangle);
             vRectangle.Width = vRectangle.Width - vRectangle.Left;
             vRectangle.Height = vRectangle.Height - vRectangle.Top;
             MoveWindow(wParam,  // 右下
                 Screen.GetWorkingArea(this).Width - vRectangle.Width, 
                 Screen.GetWorkingArea(this).Height - vRectangle.Height,
                 vRectangle.Width, vRectangle.Height, false);
             UnhookWindowsHookEx(hookHandle);
             break;
     }
     return CallNextHookEx(hookHandle, nCode, wParam, lParam);
}

private void button1_Click(object sender, EventArgs e)
{
    hookHandle = SetWindowsHookEx(WH_CBT, new HookProc(CBTHookCallback), 
        GetModuleHandle(null), 0);
    MessageBox.Show("Zswang 路过");
}

public class MessageBox
    {
        public MessageBox()
        { 
        
        }
        
        public static DialogResult Show()
        {
            Form2 frm = new Form2();//Form2是自己新建的form，外观做的和MessageBox.show出来的效果一样。设置窗体的StartPosition为CenterParent.
            return frm.ShowDialog();
        }
    }

public class MessageBox
    {
        public MessageBox()
        { 
        
        }
        
        public static DialogResult Show()
        {
            Form2 frm = new Form2();//Form2是自己新建的form，外观做的和MessageBox.show出来的效果一样。设置窗体的StartPosition为CenterParent.
            return frm.ShowDialog();
        }
    }

From2 fm = new Form2();
Point point = PointToScreen(this.Location);
fm.StartPosition = FormStartPosition.Manual;
fm.Location = new Point(point.X + (this.Size.Width - fm.Size.Width) / 2, point.Y + (this.Size.Height - fm.Size.Height) / 2);
fm.Show();