c++代码
思路是c#调摄相头拍照，将照片传给c++写的dll进行人脸识别处理再按返回的结果画框。这里c++的返回是一个字符串，以逗号分隔的坐标点，这个原型主要是为了后面做返回json串做准备的。
c++的dll项目要写一个def文件，内容为：
LIBRARY

EXPORTS
facedetect

c++代码内容为：

...
#pragma once
#ifdef DLLEXAM_EXPORTS
#define DLLEXAM_API __declspec(dllexport)
#else
#define DLLEXAM_API __declspec(dllimport)
#endif
DLLEXAM_API int facedetect(byte pImg[], int nH, int nW, int stride, char *output);
int facedetect(byte pImg[], int nH, int nW, int stride, char *output) {
   Mat image = Mat(nH, nW, CV_8UC3, pImg, stride).clone();
   dlib::cv_image<rgb_pixel> dlib_img(image);
   frontal_face_detector detector = get_frontal_face_detector();
   std::vector<rectangle> dets = detector(dlib_img); 
   int i = 0;
   std::string ret = "";
   for (int i = 0;i < dets.size() ;i++) {
   	rectangle r=dets[i];		 
   	ret.append(to_string(r.left()));
   	ret.append(",");
   	ret.append(to_string(r.top()));
   	ret.append(",");
   	ret.append(to_string(r.right()));
   	ret.append(",");
   	ret.append(to_string(r.bottom()));
   	ret.append(",");
   }	 
   strcpy(output, ret.c_str());
   return 1;	 
}

c#代码：

...
    [DllImport("DLINK.dll", EntryPoint = "facedetect", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Unicode)]
        public static extern int facedetect(byte[] bimage, int nH, int nW, int stride, ref byte output);

  Bitmap img = vispShoot.GetCurrentVideoFrame();//调摄相头拍照
            if (img == null) return;
            Bitmap bmp = null;
            //将图像转换为24位rgb图像
            if (img.PixelFormat != System.Drawing.Imaging.PixelFormat.Format24bppRgb)
            {
                 bmp = new Bitmap(img.Width, img.Height, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    g.DrawImage(img, new Rectangle(0, 0, img.Width, img.Height));
                }
            }
            else
            {
                bmp = img;
            }
            System.Drawing.Rectangle rect = new System.Drawing.Rectangle(0, 0, bmp.Width, bmp.Height);
            System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite, bmp.PixelFormat);
            //Get the address of the first line.
            IntPtr ptr = bmpData.Scan0;
            // Declare an array to hold the bytes of the bitmap. 
            int bytesLength = Math.Abs(bmpData.Stride) * bmp.Height;
            int imageWeidth = bmp.Width;
            int imageHeight = bmp.Height;
            //图像的Stride
            int imageStride = bmpData.Stride;
            byte[] buffer = new byte[bytesLength];
            // Copy the RGB values into the array.
            Marshal.Copy(ptr, buffer, 0, bytesLength);
            bmp.UnlockBits(bmpData);
            byte[] ret = new byte[1000];
            int count=0;
            count= facedetect(buffer, imageHeight, imageWeidth, imageStride,ref ret[0] );
            string strGet = System.Text.Encoding.Default.GetString(ret, 0, ret.Length); //将字节数组转换为字符串
            MemoryStream ms = new MemoryStream();
            byte[] imagedata = null;
            bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);           
            imagedata = ms.GetBuffer();

            //画个人脸框
            if (count > 0)
            {
                String[] strp = strGet.Split(new char[] { ',' });
                rectangle[] rects = new rectangle[count];
                for (int i = 0; i < count; i++)
                {
                    if (strp.Length < (i + 1) * 4) break;
                    int p = i * 4;
                    rects[i].left = Convert.ToInt32(strp[p]);
                    p++;
                    rects[i].top = Convert.ToInt32(strp[p]);
                    p++;
                    rects[i].right = Convert.ToInt32(strp[p]);
                    p++;
                    rects[i].bottom = Convert.ToInt32(strp[p]);
                }

                Graphics g = Graphics.FromImage(bmp);
                for (int i = 0; i < count; i++)
                {
                    g.DrawLine(new Pen(Color.Red, 2.0f), new Point(rects[i].left, rects[i].top), new Point(rects[i].left, rects[i].bottom));
                    g.DrawLine(new Pen(Color.Red, 2.0f), new Point(rects[i].left, rects[i].top), new Point(rects[i].right, rects[i].top));
                    g.DrawLine(new Pen(Color.Red, 2.0f), new Point(rects[i].left, rects[i].bottom), new Point(rects[i].right, rects[i].bottom));
                    g.DrawLine(new Pen(Color.Red, 2.0f), new Point(rects[i].right, rects[i].bottom), new Point(rects[i].right, rects[i].top));
                }



            }
            pictureBox1.Image = bmp;