Are you running your application or Visual Studio that hosts the app under administrative privilege?
If that's the case, the Windows prevents the drag drop operation from happening. In Windows 7 or Vista, an application with a lower security privilege (your Windows Explorer with normal user privilege) cannot send data to another one (your app or Visual Studio with administrative privilege) with a higher security privilege.
Try to do one of the followings:
Run your Visual Studio without administrative privilege. Run your application outside Visual Studio without administrative privilege. Enable and long on to the Administrator account in the OS. In this case, both your Windows Explorer and your Visual Studio will have administrative privileges.
Be aware of windows vista/windows 7 security rights - if you are running Visual Studio as administrator, you will not be able to drag files from a non-administrator explorer window into your program when you run it from within visual studio. The drag related events will not even fire! I hope this helps somebody else out there not waste hours of their life...
这是因为Windows 7的安全权限限制，导致无法激发拖拽和拖放事件，解决方案为使用Administrator用户登录，或者将Visual Studio的启动权限设置为非管理员。