Get struct from C++ dll into a C# application - c++

Question

I'm learning C# and have some experience with it. For a small project I need to implement a C++ dll into my C# app. It is a licenseplate recognition sdk.I can initialize it, so calling to this C++ code is working. But I have a problem to receive a struct back from the c++ code with strings in it. I tried a lot, read here a lot, but I don't get it working. This is C# side:
[DllImport("D:\\processor.dll", EntryPoint = "StartALPR", CallingConvention = CallingConvention.Cdecl)]
[return:MarshalAs(UnmanagedType.LPStruct)]
public static extern TMyData StartALPR(TImageSource tImageSource);
The C# struct TMyData :
[StructLayout(LayoutKind.Sequential)]
public struct TMyData
{
[MarshalAs(UnmanagedType.LPTStr)]
public string PlateString;
[MarshalAs(UnmanagedType.LPTStr)]
public string PlateXML;
[MarshalAs(UnmanagedType.LPTStr)]
public string LastError;
};
And this is the method we call to send tImageSource which contains a string with a filepath to an image to analyze.
alprResult = StartALPR(tImageSource);
The file is analyzed, so that's working. I can see the plate strings in the output of VS2015.
But I'm expecting a struct "alprResult" back as defined in TMydata, but I get an exception that the method sign is not compatible with the pinvoke sign. The only information I have is an example of how to use this dll/code in C++. This is the C++ code :
TImageSource SImageSource;
TMyData *pd;
/* Set input image */
SImageSource.MyImageFile = AnsiString(CarImage).c_str();
Memo1->Lines->Clear();
/* Starting plate detect and recognition */
pd = StartALPR(&SImageSource);
/* Standard XML result, the plate datas with numbers and positions */
Memo1->Lines->Add(pd->PlateXML);
/* last error message */
Memo2->Lines->Add(pd->LastError);
/* Best characters on plate */
Edit1->Text = pd->PlateString;
This is the C++ struct from the same example :
struct TMyData;
typedef TMyData *PMyData;
struct TMyData
{
/**
PlateString: Best license plate number of the plate group
*/
const char * PlateString;
/**
PlateXML: Plate group data in standard XML string
*/
const char * PlateXML;
/**
LastError: Laast config error ex: bad file extensions .. Default: empty
*/
const char * LastError;
};
How can I use this in C#?
Thanks in advance.

Answer 1

I actually had to solve a similar issue to this recently. I simply serialized the data into a struct and shipped it over a local zeromq socket.
//
// Weather update server in C++
// Binds PUB socket to tcp://*:5556
// Publishes random weather updates
//
// Olivier Chamoux <olivier.chamoux#fr.thalesgroup.com>
//
#include <zmq.hpp>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#if (defined (WIN32))
#include <zhelpers.hpp>
#endif
#define within(num) (int) ((float) num * random () / (RAND_MAX + 1.0))
int main () {
// Prepare our context and publisher
zmq::context_t context (1);
zmq::socket_t publisher (context, ZMQ_PUB);
publisher.bind("tcp://*:5556");
publisher.bind("ipc://weather.ipc"); // Not usable on Windows.
// Initialize random number generator
srandom ((unsigned) time (NULL));
while (1) {
int zipcode, temperature, relhumidity;
// Get values that will fool the boss
zipcode = within (100000);
temperature = within (215) - 80;
relhumidity = within (50) + 10;
// Send message to all subscribers
zmq::message_t message(20);
snprintf ((char *) message.data(), 20 ,
"%05d %d %d", zipcode, temperature, relhumidity);
publisher.send(message);
}
return 0;
}
and now the client
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using ZeroMQ;
namespace Examples
{
static partial class Program
{
public static void WUClient(string[] args)
{
//
// Weather update client
// Connects SUB socket to tcp://127.0.0.1:5556
// Collects weather updates and finds avg temp in zipcode
//
// Author: metadings
//
if (args == null || args.Length < 2)
{
Console.WriteLine();
Console.WriteLine("Usage: ./{0} WUClient [ZipCode] [Endpoint]", AppDomain.CurrentDomain.FriendlyName);
Console.WriteLine();
Console.WriteLine(" ZipCode The zip code to subscribe. Default is 72622 Nürtingen");
Console.WriteLine(" Endpoint Where WUClient should connect to.");
Console.WriteLine(" Default is tcp://127.0.0.1:5556");
Console.WriteLine();
if (args.Length < 1)
args = new string[] { "72622", "tcp://127.0.0.1:5556" };
else
args = new string[] { args[0], "tcp://127.0.0.1:5556" };
}
string endpoint = args[1];
// Socket to talk to server
using (var context = new ZContext())
using (var subscriber = new ZSocket(context, ZSocketType.SUB))
{
string connect_to = args[1];
Console.WriteLine("I: Connecting to {0}…", connect_to);
subscriber.Connect(connect_to);
/* foreach (IPAddress address in WUProxy_GetPublicIPs())
{
var epgmAddress = string.Format("epgm://{0};239.192.1.1:8100", address);
Console.WriteLine("I: Connecting to {0}…", epgmAddress);
subscriber.Connect(epgmAddress);
}
} */
// Subscribe to zipcode
string zipCode = args[0];
Console.WriteLine("I: Subscribing to zip code {0}…", zipCode);
subscriber.Subscribe(zipCode);
// Process 10 updates
int i = 0;
long total_temperature = 0;
for (; i < 20; ++i)
{
using (var replyFrame = subscriber.ReceiveFrame())
{
string reply = replyFrame.ReadString();
Console.WriteLine(reply);
total_temperature += Convert.ToInt64(reply.Split(' ')[1]);
}
}
Console.WriteLine("Average temperature for zipcode '{0}' was {1}°", zipCode, (total_temperature / i));
}
}
}
}