"Unable to find an entry point" exception when trying to import a native DLL in C#

I wanted to use a native Dll in my C# .net application as this:

public const int WFSDDESCRIPTION_LEN = 256;
public const int WFSDSYSSTATUS_LEN = 256;

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Auto)]
public struct LPWFSVERSION
{
   [MarshalAs(UnmanagedType.U4, SizeConst = 4)]
   int WVersion;
   [MarshalAs(UnmanagedType.U4, SizeConst = 4)]
   int WLowVersion;
   [MarshalAs(UnmanagedType.U4, SizeConst = 4)]
   uint WHighVerion;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = WFSDDESCRIPTION_LEN + 1)]
   public string szDescription;
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = WFSDSYSSTATUS_LEN + 1)]
   public strin开发者_开发技巧g szSystemStatus;
 }        
[DllImport("MSXFS.DLL", EntryPoint = "WFSStartUP", CharSet = CharSet.Auto, ExactSpelling = false)]
public static extern int WFSStartUP([MarshalAs(UnmanagedType.U4)]int dwVersionsRequired, ref LPWFSVERSION ver);

and here the code where I've used the imported function:

        LPWFSVERSION Ver = new LPWFSVERSION();
        WFSStartUP(0x31, ref Ver);      

the problem is that it throws an exception which says:"Unable to find an entry point"

I have dump the msxfs.dll with dumpbin and the result was this:

Section contains the following exports for MSXFS.d
00000000 characteristics
46938FBD time date stamp Tue Jul 10 17:25:09 200
    0.00 version
       1 ordinal base
      37 number of functions
      37 number of names

ordinal hint RVA      name

      1    0 00009310 WFMAllocateBuffer
      2    1 000093D0 WFMAllocateMore
       .
       .
     32   1F 00005BD0 WFSOpen
     33   20 00007C80 WFSRegister
     34   21 00008CD0 WFSSetBlockingHook
     35   22 00004FA0 WFSStartUp

It seems to be there was no problem in dll side as the dumpbin could find entry point as well. I don't know what the problem is? :(

---

 35   22 00004FA0 WFSStartUp

That's Up, not UP.