Callback function using PInvoke

[Michibeck]

Hi,

I need some help regarding callback function written in c# and called from unmanaged c code. (c dll).
The problem is that the callback function crashs after the first call with a memory access exception. I tried to protect the callback function to be handle by the GC but it doesn't work. What could I do to fix that problem??

Thanks, Michibeck

C Code:

/***********************************************************************
**  
** Function    : APH_RegTxTsHookCb
**  
** Description : Registers a callback function that hooks the 
**               Tx timestamps to the application. 
**               Only one callback function can be registered.
**
**    See Also : APH_UnregTxTsHook()
**  
** Parameter   : pf_cbTxTs (IN) - Tx timestamp callback function to register
**               
** Returnvalue : -
** 
** Remarks     : -
**  
***********************************************************************/
void APH_RegTxTsHookCb( void (k_CB_CALL_CONV *pf_cbTxTs)(PTP_t_TmStmp      s_txTs,
                                                         UINT16            w_seqId,
                                                         UINT16            w_txIfIdx,
                                                         PTP_t_msgTypeEnum e_mType));

/***********************************************************************
**  
** Function    : APH_UnregTxTsHook
**  
** Description : Unregisters the Tx timestamp hook function.
**    
**    See Also : APH_RegTxTsHookCb()
**  
***********************************************************************/
void APH_UnregTxTsHook( void );

C# Code:

 public delegate void TxTsHookCb([MarshalAs(UnmanagedType.Struct)] sTimeStamp s_rxTs, UInt16 w_seqId, UInt16 w_rxIfIdx, ePTPMsgTypes e_mType);

public class PTP
{
[DllImport("MyDll.dll")]
        static extern void APH_RegTxTsHookCb([MarshalAs(UnmanagedType.FunctionPtr)]TxTsHookCb callback);
        [DllImport("MyDll.dll")]
        static extern void APH_UnregTxTsHook();

  public static void TxHookCbFunc([MarshalAs(UnmanagedType.Struct)]sTimeStamp s_rxTs, UInt16 w_seqId, UInt16 w_rxIfIdx, ePTPMsgTypes e_mType)
        {
            Console.WriteLine(w_seqId);
            Console.WriteLine(s_rxTs.Print());
            Console.WriteLine(w_rxIfIdx);
            Console.WriteLine(e_mType);
        }

 public static sMsgTimeStamp GetTransmittingTimestamp(ePTPMsgTypes msgType, uint timeoutSec)
        {
            TxTsHookCb callbackTx = new TxTsHookCb(TxHookCbFunc); 
            int i = 0;
             
            timeStampBuffer = new sMsgTimeStamp();
            timeStampBuffer.IsSet = false;
            timeStampBuffer.srcPort = ownPortId;
            timeStampBuffer.msgType = msgType;

            APH_RegTxTsHookCb(callbackTx);

            while (!timeStampBuffer.IsSet)
            {
                System.Threading.Thread.Sleep(500);
                i++;
                if (i * 2 > timeoutSec) break;
            }

            APH_UnregTxTsHook();

            GC.KeepAlive(callbackTx); 

            return timeStampBuffer;
        }
}