C# - Detect face and crop image

You can use HaarCascade class in EmguCV (DotNet port of OpenCV) http://www.emgu.com/wiki/index.php/Face_detection

Notes in order to run this example:

• Create a Windows Form Application
• Add a PictureBox and a Timer (and Enable it) - Run it on a x86 system
• Be sure you have the OpenCV relevant dlls (included with the Emgu CV download) in the folder where you code executes.
• Adjust the path to find the Haarcascade xml (last line of the code)

using System;
using System.Windows.Forms;
using System.Drawing;
using Emgu.CV;
using Emgu.Util;
using Emgu.CV.Structure;
using Emgu.CV.CvEnum;
 
namespace opencvtut
{
    public partial class Form1 : Form
    {
                private Capture cap;
                private HaarCascade haar;
 
        public Form1()
        {
            InitializeComponent();
        }
 
        private void timer1_Tick(object sender, EventArgs e)
        {
                using (Image<Bgr, byte> nextFrame = cap.QueryFrame())
                {
                        if (nextFrame != null)
                        {
                                // there's only one channel (greyscale), hence the zero index
                                //var faces = nextFrame.DetectHaarCascade(haar)[0];
                                Image<Gray, byte> grayframe = nextFrame.Convert<Gray, byte>();
                                var faces =
                                        grayframe.DetectHaarCascade(
                                                haar, 1.4, 4,
                                                HAAR_DETECTION_TYPE.DO_CANNY_PRUNING,
                                                new Size(nextFrame.Width/8, nextFrame.Height/8)
                                                )[0];
 
                                foreach (var face in faces)
                                {
                                        nextFrame.Draw(face.rect, new Bgr(0,double.MaxValue,0), 3);
                                }
                                pictureBox1.Image = nextFrame.ToBitmap();
                        }
                }
        }
 
        private void Form1_Load(object sender, EventArgs e)
        {
            // passing 0 gets zeroth webcam
                        cap = new Capture(0);
            // adjust path to find your xml
                        haar = new HaarCascade(
                "..\\..\\..\\..\\lib\\haarcascade_frontalface_alt2.xml");
        }
    }
}

If you are looking for cropping your image, you could use the Microsoft Cognitive Service named Face API which delimiters the face of all persons on your photo, it gives you back a JSON which has the elements to return you a Rectangle struct, then you can Crop and Resize your image as you want.

Here you can see more informationa about it: FaceAPI

There's an example on CodeProject, seems like that would be a very good place to start.