callback functions in unmanaged code(urgent)

[vijaya]

I've to invoke a unmanaged dll fucntion in C# which uses
a callback fucntion.The unmanaged dll fucntion is as
follows

*************** *************** **********
The Original Fucntion in the dll
*************** *************** **********
int PBORCA_LibraryD irectory ( HPBORCA hORCASession, LPSTR
lpszLibName, LPSTR lpszLibComments , int iCmntsBuffSize,
PBORCA_LISTPROC pListProc, LPVOID pUserData );

hORCASession-Handle to previously established ORCA session

pListProc- Pointer to the PBORCA_LibraryD irectory callback
function.

pUserData -Pointer to user data to be passed to the
PBORCA_LibraryD irectory callback function

Given below function is the callback function declaration
in C.

typedef void (FAR PASCAL *PBORCA_LISTPRO C) (
PPBORCA_DIRENTR Y, LPVOID );

PPBORCA_DIRENTR Y -Pointer to the structure
PBORCA_DIRENTRY , described next
LPVOID -Long pointer to user data

The user data strcuture to the callback fucntion is as
follows:

typedef struct ORCA_UserDataIn fo
{
LPSTR lpszBuffer; // Buffer for entry info
DWORD dwCallCount; // # of entries in lib
DWORD dwBufferSize; // size of buffer
DWORD dwBufferOffset; // current offset in buffer
} ORCA_USERDATAIN FO, FAR *PORCA_USERDATA INFO;

The structure passed to the callback fucntion by the
LibraryDirector y fucntion is as follows:

typedef struct pborca_direntry { CHAR szComments
[PBORCA_MAXCOMME NT + 1];
LONG lCreateTime;
LONG lEntrySize;
LPSTR lpszEntryName;
PBORCA_TYPE otEntryType;} PBORCA_DIRENTRY , FAR
*PPBORCA_DIRENT RY;

Note:PBORCA_TYP E is an enum
PBORCA_MAXCOMME NT=255
*************** *************** *********
End of the dll fucntion
*************** *************** *********

_______________ _______________ ___________
C# code which I've written
_______________ _______________ ___________

[StructLayout(La youtKind.Sequen tial,CharSet=Ch arSet.Auto)]
public class UserData
{
public string s;
public uint callcount;
public uint bufsize;
public uint offset;
}
[StructLayout(La youtKind.Sequen tial,CharSet=Ch arSet.Auto)]
public struct pborca_direntry
{
public const int i=255;
[MarshalAsUnmana gedType.ByValTS tr,SizeConst=i+ 1)]
public string s;
public System.Int32 time;
public System.Int32 size;
[MarshalAs(Unman agedType.LPStr)]
public string name;
public int type;
}
public unsafe delegate void Callback(ref pborca_direntry
p,UserData usr);

[DllImport
("pborc80.dll", CharSet=CharSet .Auto,EntryPoin t="PBORCA_Lib r
aryDirectory")]
public static extern int LibraryEntry(in t hnd,[MarshalAs
(UnmanagedType. LPStr)]System.String s,[Out]char[] buff,int
se,[MarshalAs(Unman agedType.Functi onPtr)]Callback
hr,UserData usr);


public class1
{
Callback c = new Callback(Class1 .libdircallback );
char[] st = new char[10000];
GC.KeepAlive(co de=class3.Libra ryEntry
(hnd,"d:\\proje cts\\current\\t gt3.pbl",st,100 00,c,usr));

public static void libdircallback( ref pborca_direntry
s,UserData usr)
{
Console.WriteLi ne("hai,I'm in callback");
Console.WriteLi ne(s.type);
}

}

Error: The return code is 0 which means the fucntion has
executed successfully. But I'm not getting the correct
values in the structure pborcadirentry.

Note: I've tried GC method. But I'm not sure whether I've
applied it appropriately.I 've also tried [MarshalAs
(UnmanagedType. Struct)] and [MarshalAs
(UnmanagedType. LPStruct)] for marshaling structure and
class.
But there was no change in the o/p. I feel that there is
some problem with the memory. But I'm unable to detect it.
Please help me

Thanks in advance,
Vijaya
..