C#

public partial class Form1 : Form
    {
        bool canResize;
        int mX, mY;
        Rectangle rect = new Rectangle();

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.DrawRectangle(new Pen(Brushes.Purple, 2), rect);
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            canResize = true;
            mX = e.X; mY = e.Y;

            rect.Location = new Point(mX, mY);
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            canResize = false;
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (canResize)
            {
                if (mX < e.X)
                {
                    rect.X = mX;
                    rect.Width = e.X - mX;
                }
                else
                {
                    rect.X = e.X;
                    rect.Width = mX - e.X;
                }
                if (mY < e.Y)
                {
                    rect.Y = mY;
                    rect.Height = e.Y - mY;
                }
                else
                {
                    rect.Y = e.Y;
                    rect.Height = mY - e.Y;
                }
                this.Invalidate();
            }
        }
    }