WinForm中管理员许可权下获取拖拽文件路径的解决方案
今天某个群里有人问为什么拖拽文件到程序里的操作无效。正好我遇到过这个问题，就翻出来自己以前写的代码，改成一个类，算做这个问题的解决方案。
导致拖拽无效的原因可以看这篇文章：Q: Why Doesn』t Drag-and-Drop work when my Application is Running Elevated? - A: Mandatory Integrity Control and UIPI ，其中也给出了解决的方法，但对于WinForm而言不太够，因为需要处理其他额外的细节。
我已经忘记当初我写这些代码时的心路历程了，所以就不仔细分析，就说关键的内容：
- 使程序能够接收相关消息。
- 对WinForm而言会以自己的方式处理拖拽，所以控制项启用了AllowDrop属性，则无法接收WM_DROPFILES消息。
- 同第2条，即使程序可以接收消息，也需要手动处理消息和引发事件。这里我选择的时使用Control.DoDragDrop()方法。
其他更多细节见代码：
public sealed class FileDropHandler : IMessageFilter, IDisposable
{
#region native members
[DllImport("user32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern bool ChangeWindowMessageFilterEx(IntPtr hWnd, uint message, ChangeFilterAction action, in ChangeFilterStruct pChangeFilterStruct);
[DllImport("shell32.dll", SetLastError = false, CallingConvention = CallingConvention.Winapi)]
private static extern void DragAcceptFiles(IntPtr hWnd, bool fAccept);
[DllImport("shell32.dll", SetLastError = false, CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Winapi)]
private static extern uint DragQueryFile(IntPtr hWnd, uint iFile, StringBuilder lpszFile, int cch);
[DllImport("shell32.dll", SetLastError = false, CallingConvention = CallingConvention.Winapi)]
private static extern void DragFinish(IntPtr hDrop);
[StructLayout(LayoutKind.Sequential)]
private struct ChangeFilterStruct
{
public uint CbSize;
public ChangeFilterStatu ExtStatus;
}
private enum ChangeFilterAction : uint
{
MSGFLT_RESET,
MSGFLT_ALLOW,
MSGFLT_DISALLOW
}
private enum ChangeFilterStatu : uint
{
MSGFLTINFO_NONE,
MSGFLTINFO_ALREADYALLOWED_FORWND,
MSGFLTINFO_ALREADYDISALLOWED_FORWND,
MSGFLTINFO_ALLOWED_HIGHER
}
private const uint WM_COPYGLOBALDATA = 0x0049;
private const uint WM_COPYDATA = 0x004A;
private const uint WM_DROPFILES = 0x0233;
#endregion
private const uint GetIndexCount = 0xFFFFFFFFU;
private Control _ContainerControl;
private readonly bool _DisposeControl;
public Control ContainerControl { get; }
public FileDropHandler(Control containerControl) : this(containerControl, false) { }
public FileDropHandler(Control containerControl, bool releaseControl)
{
_ContainerControl = containerControl ?? throw new ArgumentNullException("control", "control is null.");
if (containerControl.IsDisposed) throw new ObjectDisposedException("control");
_DisposeControl = releaseControl;
var status = new ChangeFilterStruct() { CbSize = 8 };
if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_DROPFILES, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_COPYGLOBALDATA, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
if (!ChangeWindowMessageFilterEx(containerControl.Handle, WM_COPYDATA, ChangeFilterAction.MSGFLT_ALLOW, in status)) throw new Win32Exception(Marshal.GetLastWin32Error());
DragAcceptFiles(containerControl.Handle, true);
Application.AddMessageFilter(this);
}
public bool PreFilterMessage(ref Message m)
{
if (_ContainerControl == null || _ContainerControl.IsDisposed) return false;
if (_ContainerControl.AllowDrop) return _ContainerControl.AllowDrop = false;
if (m.Msg == WM_DROPFILES)
{
var handle = m.WParam;
var fileCount = DragQueryFile(handle, GetIndexCount, null, 0);
var fileNames = new string[fileCount];
var sb = new StringBuilder(262);
var charLength = sb.Capacity;
for (uint i = 0; i < fileCount; i++)
{
if (DragQueryFile(handle, i, sb, charLength) > 0) fileNames[i] = sb.ToString();
}
DragFinish(handle);
_ContainerControl.AllowDrop = true;
_ContainerControl.DoDragDrop(fileNames, DragDropEffects.All);
_ContainerControl.AllowDrop = false;
return true;
}
return false;
}
public void Dispose()
{
if (_ContainerControl == null)
{
if (_DisposeControl && !_ContainerControl.IsDisposed) _ContainerControl.Dispose();
Application.RemoveMessageFilter(this);
_ContainerControl = null;
}
}
}
这个类虽不优雅，但用法很简单：
public FileDropHandler FileDroper; //全局的
private void Form1_Load(object sender, EventArgs e)
{
FileDroper = new FileDropHandler(label1); //初始化
}
//这是 label1 的DragEnter事件。
private void label1_DragEnter(object sender, DragEventArgs e)
{
var paths = e.Data.GetData(typeof(string[]));
}
这个类是专供WinForm用的，虽然很多人说WinForm过时了，然而我还是喜欢用它写小工具。这个类很容易就改成WPF版的，这里不赘述。自己查资料即可，关键词是「消息」。
当然如果你觉得这个类太糙，也可以用复杂的方式处理消息，自己搞一套事件，或者自己实现一个IDataObject类型来存数据……
此代码基于WTFPL发布。
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