前言
最近写的一个小工具因涉及到系统文件的操作，故需要以管理员身份运行，但这影响到了程序的文件拖放功能，即DragDrop事件。而一番搜索后，发现这个问题还是令很多人感到棘手的，而大部分网友给出的解决方法均是效果不佳。很幸运的是我发现了一个很稳的解决方法，这里做个记录。
原因
不同权限提升级别的程序之间是无法共享拖放消息的，这就导致了例如RequireAdministrator权限提升级别的程序读取不到InvokeAsUser权限提升级别的Windows Explorer中的文件数据的问题。即低权限的进程无法向高权限的进程发送任何高于WM_USER的消息，而低于WM_USER 的消息一部分也会因为安全原因被禁止。故同样级别的权限提升账户运行的程序则可以共享拖放消息。
解决
综合原因分析，可以得知通常所用的AllowDrop只限于同级别权限的程序之间共享消息。故要解决这个问题则可以使用传统的拖放方法，即WndProc函数中的消息。
这里需要注意的是，此解决方案无需启用控件的AllowDrop属性。
类
public class ElevatedDragDropManager : IMessageFilter
{
#region "P/Invoke"
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool ChangeWindowMessageFilterEx(IntPtr hWnd, uint msg, ChangeWindowMessageFilterExAction action, ref CHANGEFILTERSTRUCT changeInfo);
[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool ChangeWindowMessageFilter(uint msg, ChangeWindowMessageFilterFlags flags);
[DllImport("shell32.dll")]
private static extern void DragAcceptFiles(IntPtr hwnd, bool fAccept);
[DllImport("shell32.dll")]
private static extern uint DragQueryFile(IntPtr hDrop, uint iFile, [Out()]
StringBuilder lpszFile, uint cch);
[DllImport("shell32.dll")]
private static extern bool DragQueryPoint(IntPtr hDrop, ref POINT lppt);
[DllImport("shell32.dll")]
private static extern void DragFinish(IntPtr hDrop);
[StructLayout(LayoutKind.Sequential)]
private struct POINT
{
public int X;
public int Y;
public POINT(int newX, int newY)
{
X = newX;
Y = newY;
}
public static implicit operator System.Drawing.Point(POINT p)
{
return new System.Drawing.Point(p.X, p.Y);
}
public static implicit operator POINT(System.Drawing.Point p)
{
return new POINT(p.X, p.Y);
}
}
private enum MessageFilterInfo : uint
{
None,
AlreadyAllowed,
AlreadyDisAllowed,
AllowedHigher
}
private enum ChangeWindowMessageFilterExAction : uint
{
Reset,
Allow,
Disallow
}
private enum ChangeWindowMessageFilterFlags : uint
{
Add = 1,
Remove = 2
}
[StructLayout(LayoutKind.Sequential)]
private struct CHANGEFILTERSTRUCT
{
public uint cbSize;
public MessageFilterInfo ExtStatus;
}
#endregion
public static ElevatedDragDropManager Instance = new ElevatedDragDropManager();
public event EventHandler<ElevatedDragDropArgs> ElevatedDragDrop;
private const uint WM_DROPFILES = 0x233;
private const uint WM_COPYDATA = 0x4a;
private const uint WM_COPYGLOBALDATA = 0x49;
private readonly bool IsVistaOrHigher = Environment.OSVersion.Version.Major >= 6;
private readonly bool Is7OrHigher = (Environment.OSVersion.Version.Major == 6 && Environment.OSVersion.Version.Minor >= 1) || Environment.OSVersion.Version.Major > 6;
protected ElevatedDragDropManager()
{
Application.AddMessageFilter(this);
}
public void EnableDragDrop(IntPtr hWnd)
{
if (Is7OrHigher)
{
CHANGEFILTERSTRUCT changeStruct = new CHANGEFILTERSTRUCT();
changeStruct.cbSize = Convert.ToUInt32(Marshal.SizeOf(typeof(CHANGEFILTERSTRUCT)));
ChangeWindowMessageFilterEx(hWnd, WM_DROPFILES, ChangeWindowMessageFilterExAction.Allow, ref changeStruct);
ChangeWindowMessageFilterEx(hWnd, WM_COPYDATA, ChangeWindowMessageFilterExAction.Allow, ref changeStruct);
ChangeWindowMessageFilterEx(hWnd, WM_COPYGLOBALDATA, ChangeWindowMessageFilterExAction.Allow, ref changeStruct);
}
else if (IsVistaOrHigher)
{
ChangeWindowMessageFilter(WM_DROPFILES, ChangeWindowMessageFilterFlags.Add);
ChangeWindowMessageFilter(WM_COPYDATA, ChangeWindowMessageFilterFlags.Add);
ChangeWindowMessageFilter(WM_COPYGLOBALDATA, ChangeWindowMessageFilterFlags.Add);
}
DragAcceptFiles(hWnd, true);
}
public bool PreFilterMessage(ref Message m)
{
if (m.Msg == WM_DROPFILES)
{
HandleDragDropMessage(m);
return true;
}
return false;
}
private void HandleDragDropMessage(Message m)
{
dynamic sb = new StringBuilder(260);
uint numFiles = DragQueryFile(m.WParam, 0xffffffffu, sb, 0);
dynamic list = new List<string>();
for (uint i = 0; i <= numFiles - 1; i++)
{
if (DragQueryFile(m.WParam, i, sb, Convert.ToUInt32(sb.Capacity) * 2) > 0)
{
list.Add(sb.ToString());
}
}
POINT p = default(POINT);
DragQueryPoint(m.WParam, ref p);
DragFinish(m.WParam);
dynamic args = new ElevatedDragDropArgs();
args.HWnd = m.HWnd;
args.Files = list;
args.X = p.X;
args.Y = p.Y;
if (ElevatedDragDrop != null)
{
ElevatedDragDrop(this, args);
}
}
}
public class ElevatedDragDropArgs : EventArgs
{
public IntPtr HWnd
{
get { return m_HWnd; }
set { m_HWnd = value; }
}
private IntPtr m_HWnd;
public List<string> Files
{
get { return m_Files; }
set { m_Files = value; }
}
private List<string> m_Files;
public int X
{
get { return m_X; }
set { m_X = value; }
}
private int m_X;
public int Y
{
get { return m_Y; }
set { m_Y = value; }
}
private int m_Y;
public ElevatedDragDropArgs()
{
Files = new List<string>();
}
}使用示例
private void Form1_ElevatedDragDrop(System.Object sender, ElevatedDragDropArgs e)
{
// Add the files to listview
if (e.HWnd == listView1.Handle)
{
foreach (string file in e.Files)
{
listView1.Items.Add(file);
}
}
}
private void Form1_Load(object sender, EventArgs e)
{
ElevatedDragDropManager.Instance.EnableDragDrop(listView1.Handle);
// Enable elevated drag drop on listView1. Note that I used the Handle property
ElevatedDragDropManager.Instance.ElevatedDragDrop += Form1_ElevatedDragDrop;
}