c# win app forms detect the number of faces on image and prompt error message if there is more than 1 face

I am currently doing an ID pic uploading system using C# Windows App Forms and I would like to allow the user to upload an image and the image must only contain 1 front face. To prevent user from uploading more than 1 face, I would like to prompt them an error message once the system detects more than one face in an image but I am not sure how to go about it. I used takuya takeuchi's dlibdotnet library.

Here is my current code.

namespace DetectTrial

{

public partial class Form1 : Form  {      #region Fields      private readonly BackgroundWorker _BackgroundWorker;