ReadDirectoryChangesW

C# Signature:

[DllImport("kernel32.dll")]
static extern bool ReadDirectoryChangesW(IntPtr hDirectory, IntPtr lpBuffer,
   uint nBufferLength, bool bWatchSubtree, uint dwNotifyFilter, out uint
   lpBytesReturned, IntPtr lpOverlapped,
   ReadDirectoryChangesDelegate lpCompletionRoutine);

Alternative Managed API:

System.IO.FileSystemWatcher