This example show how to building a Xamarin.Forms applications using Microsoft Cognitive Services to identify if two pictures are from same person - http://julianocustodio.com/face-verify

MainPage.FaceVerify.cs

            public async void FaceVerify(object sender, EventArgs e)
	    {
	        if (faceId1 == new Guid())
	            await FaceAddInPerson(filePath1);

                // Call the Face API.
                try
	        {
                    var result = await _faceServiceClient.VerifyAsync(faceId2,"grupo",personId);


	            await DisplayAlert("Result", 
                                   "IsIdentical: " + result.IsIdentical +
                                   " Confidence: " + result.Confidence, 
                                   "OK");

	            await _faceServiceClient.DeletePersonFaceAsync("grupo", personId, faceId1);
                    faceId1 = new Guid();

                 }
	         // Catch and display Face API errors.
	         catch (FaceAPIException f)
	         {
	         await DisplayAlert("Error", f.ErrorMessage, "ok");
	         }
	         // Catch and display all other errors.
	         catch (Exception ex)
	         {
	             await DisplayAlert("Error", ex.Message, "ok");
	         }
	    }