I do not have access to the Powersoft OpenLibrary API DLL
<http://sybooks.sybase.com/onlinebooks/group-pb/adt0650e/orca/>, so I
created a fake replacement for it:

---- Begin ORCA.DLL ----------------------------------------------------

#define PBORCA_MAXCOMMENT 255

typedef int PBORCA_TYPE;

typedef void *HPBORCA;

typedef struct pborca_direntry {
    CHAR szComments[PBORCA_MAXCOMMENT + 1];
    LONG lCreateTime;
    LONG lEntrySize;
    LPSTR lpszEntryName;
    PBORCA_TYPE otEntryType;
} PBORCA_DIRENTRY, FAR *PPBORCA_DIRENTRY;

typedef void (FAR PASCAL *PBORCA_LISTPROC) ( PPBORCA_DIRENTRY, LPVOID );

extern "C" __declspec(dllexport) int PBORCA_LibraryDirectory ( HPBORCA
hORCASession, LPSTR lpszLibName, LPSTR lpszLibComments, int
iCmntsBuffSize, PBORCA_LISTPROC pListProc, LPVOID pUserData );

int PBORCA_LibraryDirectory ( HPBORCA hORCASession, LPSTR lpszLibName,
LPSTR lpszLibComments, int iCmntsBuffSize, PBORCA_LISTPROC pListProc,
LPVOID pUserData )
{
    strncpy(lpszLibComments, "These are the library comments.",
iCmntsBuffSize);
    pborca_direntry entry;
    entry.lCreateTime = 1;
    entry.lEntrySize = 100;
    entry.lpszEntryName = "Entry 1";
    entry.otEntryType = 3;
    strcpy(entry.szComments, "These are the comments for entry one.");
    pListProc(&entry, pUserData);
    entry.lCreateTime = 2;
    entry.lEntrySize = 200;
    entry.lpszEntryName = "Entry 2";
    entry.otEntryType = 4;
    strcpy(entry.szComments, "These are the comments for entry two.");
    pListProc(&entry, pUserData);
    return 0;
}

---- End ORCA.DLL ------------------------------------------------------

The following C# executable works perfectly with ORCA.DLL.

---- Begin ORCAClient.EXE ----------------------------------------------

using System;
using System.Text;
using System.Runtime.InteropServices;

namespace ORCAClient
{
    [StructLayout(LayoutKind.Sequential)]
    public sealed class DirectoryEntry
    {
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=256)]
        public string Comments;
        public int CreateTime;
        public int EntrySize;
        public string EntryName;
        public int EntryType; // int or short?

        public void WriteToConsole()
        {
            Console.WriteLine("Comments: " + Comments);
            Console.WriteLine("CreateTime: " + CreateTime);
            Console.WriteLine("EntrySize: " + EntrySize);
            Console.WriteLine("EntryName: " + EntryName);
            Console.WriteLine("EntryType: " + EntryType);
        }
    }

    public delegate void DirectoryCallback(DirectoryEntry entry, IntPtr
userData);

    public sealed class ORCASession
    {
        ORCASession() {}

        [DllImport("ORCA.DLL")]
        public static extern int PBORCA_LibraryDirectory(IntPtr sessionHandle,
string libraryName, StringBuilder libraryCommentsBuffer, int
libraryCommentsBufferSize, DirectoryCallback callback, IntPtr userData);
    }

    class Test
    {
        static void TestCallback(DirectoryEntry entry, IntPtr userData)
        {
            Console.WriteLine("Callback called.");
            entry.WriteToConsole();
        }

        static void Main(string[] args)
        {
            StringBuilder comments = new StringBuilder(256);
            int result = ORCASession.PBORCA_LibraryDirectory(IntPtr.Zero,
"MyLibrary", comments, comments.Capacity, new
DirectoryCallback(TestCallback), IntPtr.Zero);
            Console.WriteLine("Result: " + result);
            Console.WriteLine("Library comments: " + comments);
            Console.ReadLine();
        }
    }
}

---- End ORCAClient.EXE ------------------------------------------------

Note that you do not need to use GC.KeepAlive to keep your delegate
object alive, since it is automatically kept alive by P/Invoke for the
duration of the PBORCA_LibraryDirectory call.

Note also that the userData value is not very useful for managed code.
It is easier and more statically typed to pass an instance method as the
callback delegate. The target object of the delegate serves as user
data. See the following example:

---- Begin ORCAClient.EXE Version 2 ------------------------------------

using System;
using System.Collections;
using System.Text;
using System.Runtime.InteropServices;

namespace ORCAClient
{
    [StructLayout(LayoutKind.Sequential)]
    sealed class DirectoryEntry
    {
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst=256)]
        public string Comments;
        public int CreateTime;
        public int EntrySize;
        public string EntryName;
        public int EntryType; // int or short?

        public void WriteToConsole()
        {
            Console.WriteLine("Comments: " + Comments);
            Console.WriteLine("CreateTime: " + CreateTime);
            Console.WriteLine("EntrySize: " + EntrySize);
            Console.WriteLine("EntryName: " + EntryName);
            Console.WriteLine("EntryType: " + EntryType);
        }
    }

    delegate void DirectoryCallback(DirectoryEntry entry, IntPtr userData);

    sealed class ORCASession
    {
        ORCASession() {}

        [DllImport("ORCA.DLL")]
        public static extern int PBORCA_LibraryDirectory(IntPtr sessionHandle,
string libraryName, StringBuilder libraryCommentsBuffer, int
libraryCommentsBufferSize, DirectoryCallback callback, IntPtr userData);
    }

    class DirectoryListing
    {
        ArrayList entries = new ArrayList();

        public void MyDirectoryCallback(DirectoryEntry entry, IntPtr userData)
        {
            entries.Add(entry);
        }

        public void WriteToConsole()
        {
            foreach(DirectoryEntry entry in entries)
            {
                entry.WriteToConsole();
            }
        }
    }

    class Test
    {
        static void Main(string[] args)
        {
            DirectoryListing listing = new DirectoryListing();
            StringBuilder comments = new StringBuilder(256);
            int result = ORCASession.PBORCA_LibraryDirectory(IntPtr.Zero,
"MyLibrary", comments, comments.Capacity, new
DirectoryCallback(listing.MyDirectoryCallback), IntPtr.Zero);
            Console.WriteLine("Result: " + result);
            Console.WriteLine("Library comments: " + comments);
            listing.WriteToConsole();
            Console.ReadLine();
        }
    }
}

---- End ORCAClient.EXE Version 2 --------------------------------------

Hope this helps...