C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
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;
using System.Drawing;

namespace rectTest
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }

        System.Drawing.Rectangle myRec;
        System.Windows.Point firstPoint;

        bool flg = false;
        private void Window_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            flg = true;
            firstPoint = e.GetPosition(mainwin);
            myRec = new System.Drawing.Rectangle((int)firstPoint.X, (int)firstPoint.Y, 0, 0);
        }

        private void mainwin_MouseMove(object sender, MouseEventArgs e)
        {
            if (flg == true)
            {
                System.Windows.Point currentPoint;
                currentPoint = e.GetPosition(mainwin);

                double width = currentPoint.X - firstPoint.X;
                double height = currentPoint.Y - firstPoint.Y;

                if (width > 0 && height > 0)
                {
                    Graphics g = Graphics.FromHwnd(new System.Windows.Interop.WindowInteropHelper(this).Handle);
                    System.Drawing.Pen myPen = new System.Drawing.Pen(System.Drawing.Color.Red, 5);
                    g.DrawRectangle(myPen, myRec);
                    myRec.Width = (int)width;
                    myRec.Height = (int)height;
                    InvalidateVisual();
                }
            }

        }

        private void mainwin_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            flg = false;
        }
    }
}