[DllImport("kernel32.dll")]
public static extern int DeviceIoControl(IntPtr hDevice, int
    dwIoControlCode, ref short lpInBuffer, int nInBufferSize, IntPtr
    lpOutBuffer, int nOutBufferSize, ref int lpBytesReturned, IntPtr
    lpOverlapped);
 
public static bool UncompressFile(string fileName)
{
    int lpBytesReturned = 0;
    int FSCTL_SET_COMPRESSION = 0x9C040;
    short COMPRESSION_FORMAT_NONE = 0;
 
    FileStream f = File.Open(fileName, System.IO.FileMode.Open,
        System.IO.FileAccess.ReadWrite, System.IO.FileShare.None);
 
    int result = DeviceIoControl(f.Handle, FSCTL_SET_COMPRESSION,
        ref COMPRESSION_FORMAT_NONE, 2 /*sizeof(short)*/, IntPtr.Zero, 0,
        ref lpBytesReturned, IntPtr.Zero);
 
    f.Close();
 
    return result == 1;
}

Mit freundlichen Gr??en,
Andy G.