//窗体移动API
[DllImport("user32.dll")]
public static extern bool ReleaseCapture();
[DllImport("user32.dll")]
public static extern bool SendMessage(IntPtr hwnd, int wMsg, int wParam, int IParam);
public const int WM_SYSCOMMAND = 0x0112;
public const int SC_MOVE = 0xF010;
public const int HTCAPTION = 0x0002;
[DllImport("user32")]
private static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, IntPtr lParam);
private const int WM_SETREDRAW = 0xB;



private void Form1_MouseDown(object sender, MouseEventArgs e)
{
    if (this.WindowState == FormWindowState.Normal)
    {
        ReleaseCapture();
        SendMessage(this.Handle, WM_SYSCOMMAND, SC_MOVE + HTCAPTION, 0);
    }
}

1、添加命名空间：
using System.Runtime.InteropServices;  

2、定义常量值及函数：
private const int CS_DropSHADOW = 0x20000;  
private const int GCL_STYLE = (-26);  
[DllImport("user32.dll", CharSet = CharSet.Auto)]  
public static extern int SetClassLong(IntPtr hwnd, int nIndex, int dwNewLong);  
[DllImport("user32.dll", CharSet = CharSet.Auto)]  
public static extern int GetClassLong(IntPtr hwnd, int nIndex);


3、构造方法下引用：
SetClassLong(this.Handle, GCL_STYLE, GetClassLong(this.Handle, GCL_STYLE) | CS_DropSHADOW);