kk/Captcha/MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace Captcha
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Bitmap bitmap = new Bitmap(100, 100);
        private string cd = string.Empty;
        public MainWindow()
        {
            InitializeComponent();
            cd = "553Fa";
            SetBtm(100, 100);


        }


        private void SetBtm(int w, int h)
        {
            var random = new Random();
            var pixels = new byte[256 * 256 * 4];
            random.NextBytes(pixels);
            BitmapSource bitmapSource = BitmapSource.Create(256, 256, 96, 96, PixelFormats.Bgra32, null, pixels, 256 * 4);
            var visual = new DrawingVisual();
            using (DrawingContext drawingContext = visual.RenderOpen())
            {


                drawingContext.DrawText(
                    new FormattedText("Hi!", CultureInfo.InvariantCulture, FlowDirection.LeftToRight,
                        new Typeface("Segoe UI"), 132, System.Windows.Media.Brushes.Black), new System.Windows.Point(0, 0));
                drawingContext.DrawImage(bitmapSource, new Rect(0, 0, 256, 256));
            }
            var image = new DrawingImage(visual.Drawing);
            grf.Source = image;


        }
    }
}