Cannot capture webcam using dll created by C++ in a C# application

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I am going to create a C# application to capture webcam using a dll created by C++. This dll allow me to access OpenCV library. At the moment, I use Visual Studio 2013 Expression and OpenCV 2.49 version. I have already succeeded in capturing Webcam in C++ program. When I build this into dll and read this dll from C# code, an error occurred. The error content is:

An unhandled exception of type 'System.AccessViolationException' occurred in Webcam.exe

Additional Information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt. 

The dll code I built in C++ likes this.

#include <iostream>
#include <Windows.h>
#include <opencv2\highgui\highgui.hpp>
#include <opencv2\core\core.hpp>

typedef struct _IplImageCs
{
    int width;
    int height;
    int imageSize;
}
IplImageCs;

typedef IplImageCs* pIplImageCs;

extern "C"
{
    _declspec(dllexport) int CaptureFromCamera(int num, pIplImageCs imgCs);
    _declspec(dllexport) void QueryFrame(LPBYTE imageData);
    _declspec(dllexport) void ReleaseCaptureCamera(void);
}

IplImage* img;
CvCapture* capture;
pIplImageCs imgCs;

_declspec(dllexport) int CaptureFromCamera(int num, pIplImageCs imgCs)
{
    //capture = cvCaptureFromCAM(num);
    capture = cvCreateCameraCapture(num);
    if (capture == NULL)
        return -1;
    img = cvQueryFrame(capture);
    imgCs->width = img->width;
    imgCs->height = img->height;
    imgCs->imageSize = img->imageSize;
    return 0;
}

_declspec(dllexport) void QueryFrame(LPBYTE imageData)
{
    if (capture == NULL)
        return;
    img = cvQueryFrame(capture);
    memcpy(imageData, img->imageData, img->imageSize);
}

_declspec(dllexport) void ReleaseCaptureCamera(void)
{
    cvReleaseCapture(&capture);
}

In the C#, I wrote this.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Imaging;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Webcam
{
    public partial class Form1 : Form
    {
        private bool bContinue;
        private bool bCompleted;
        private BackgroundWorker bw;
        private Bitmap bmp;

        [StructLayout(LayoutKind.Sequential)]
        private struct IplImage
        {
            public int width;
            public int height;
            public int imageSize;
            public IplImage(int init)
            {
                width = init;
                height = init;
                imageSize = init;
            }
        }
        IplImage img;

        [DllImport("Show Camera.dll", CallingConvention = CallingConvention.Cdecl)]
        static extern int CaptureFromCamera(int num, ref IplImage img);
        [DllImport("Show Camera.dll", CallingConvention = CallingConvention.Cdecl)]
        static extern void QueryFrame(Byte[] imageData);
        [DllImport("Show Camera.dll", CallingConvention = CallingConvention.Cdecl)]
        static extern void ReleaseCaptureCamera();

        public Form1()
        {
            InitializeComponent();
            panel2.AutoScroll = true;
            bw = new BackgroundWorker();
            bw.DoWork += new DoWorkEventHandler(Processing);
            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(CompletedProcessing);
            bCompleted = true;
            img = new IplImage(0);
        }

        private void ShowBt_Click(object sender, EventArgs e)
        {
            try
            {
                CaptureFromCamera(0, ref img);
                //if (CaptureFromCamera(0, ref img) < 0)
                //    return;
                int width = img.width;
                int height = img.height;
                pb.Size = new Size(width, height);
                this.ClientSize = new Size(panel1.Width + panel2.Width, panel2.Height);
                bContinue = true;
                bCompleted = false;
                bw.RunWorkerAsync();
                ShowBt.Enabled = false;
            }
            catch(Exception error)
            {
                MessageBox.Show(error.Message);
            }
        }

        private void Processing(object sender,DoWorkEventArgs e)
        {
            try
            {
                if (CaptureFromCamera(0, ref img) < 0)
                    return;
                this.Cursor = Cursors.WaitCursor;

                Byte[] imageData = new Byte[img.imageSize];

                while (bContinue)
                {
                    QueryFrame(imageData);
                    bmp = new Bitmap(img.width, img.height, PixelFormat.Format24bppRgb);
                    Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
                    BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);
                    IntPtr ptr = bmpData.Scan0;
                    Marshal.Copy(imageData, 0, ptr, img.imageSize);
                    bmp.UnlockBits(bmpData);
                    pb.Image = bmp;
                    bmp.Dispose();
                }
            }
            catch(Exception error)
            {
                MessageBox.Show(error.Message);
            }
        }

        private void CompletedProcessing(object sender, RunWorkerCompletedEventArgs e)
        {
            bCompleted = true;
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            bContinue = false;
            while (bCompleted != true)
                Application.DoEvents();
        }

        private void QuitBt_Click(object sender, EventArgs e)
        {
            Close();
        }
    }
}

I don't know how to correct the code to make it work correcty. Could anyone help me to fix it? Thank you very much for reading my problem and help me. Have a nice weekend, all!

asked 2 mins ago

nozomi254

1

Cannot capture webcam using dll created by C++ in a C# application