error on calling DISMMountImage function from c#

[poooooo]

hello,
I wanted to call dismapi.dll function in c# code.
when I call function DismInitialize( ) its working fine.
next step I want to mount .Wim image file using DismMountImage( ) function.I got error 122,unable to mount image

int hr="Marshal.GetLastWin32Error()"


I am calling DismMountImage as-

 [DllImport(@"C:\Windows\System32\CompatTel\dismapi.dll", CharSet = CharSet.Unicode, SetLastError = true)]

 [return: MarshalAs(UnmanagedType.Error)]

  public static extern int DismMountImage(
  [MarshalAs(UnmanagedType.LPWStr)] string ImageFilePath,
  [MarshalAs(UnmanagedType.LPWStr)] string MountPath,
  UInt32 ImageIndex,
  [MarshalAs(UnmanagedType.LPWStr)] string ImageName,
  DismImageIdentifier ImageIdentifier,
   UInt32 Flags
             
            );

what is wrong in this???
any help appreciated..
Thank You