Interop Question: C DLL to C#

Hello, 

I'm a beginner with interop services and having some difficulty getting it working. 

I'm hoping someone can take a look at what I've done and critique it.

Basically pass in a card number in the request and get information back from the response.

When I pass in the card number the log file tells me that I did not pass in a card number even though it is in the object. 

It must be how I am passing in the cardRequest object that is not correct but I'm not entirely sure how to proceed.

Thanks. 

C syntax:

int Card_Transactions(int transType, pCardRequest cardRequest, pCardResponse cardResponse, int (*Com)(char *));

C Structure - Request:

typedef struct _CardRequest
{
char CardNumber[80];
long Balance;
}
CardRequest, *pCardRequest

C structure - response:

typedef struct _CardResponse
{
char Result;
char Display[41];
char Date[11];
char Time[9];
char RespCode[4];
char CardNum[20];
char ExpDate[5];
long InitialBalance;
long RemainingBalance;
} CardResponse, *pCardResponse

This is what I've done:

[StructLayout(LayoutKind.Sequential)]
public struct _CardRequest
{
   [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
   public string CardNumber;
   public uint Balance;
}

[StructLayout(LayoutKind.Sequential)]
public struct _CardResponse
{
  public byte Result;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 41)]
  public string Display;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 11)]
  public string Date;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 9)]
  public string Time;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 4)]
  public string RespCode;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
  public string CardNum;

  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 5)]
  public string ExpDate;

  public uint InitialBalance;
  
  public uint RemainingBalance;

}

Then I set it up as:

[DllImport("cdcauth.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
public static extern int CardTransactions(int transactionType, ref CardRequest cardRequest, ref CardResponse cardResponse, IntPtr com);

So in my method it looks like:

CardRequest cardRequest = new CardRequest();
CardResponse cardResponse = new CardResponse();

cardRequest.CardNumber = "546732723487600543872";

int success = CardTransactions(7, ref cardRequest, ref cardResponse, IntPtr.Zero);

Sucess will be 0 which means fail. 

The log file the DLL generates says that a card number was not passed.

So i'm not sure if I set up the CardRequest object correctly or not. 

Thanks for taking a look.