C#

Rectangle re;
 private void form_MouseDown(object sender, MouseEventArgs e)
 {
     re.X = e.X;
     re.Y = e.Y;
     re.Width = 0;
     re.Height = 0;
 }

 private void form_MouseMove(object sender, MouseEventArgs e)
 {
     if (e.Button == MouseButtons.Left)
     {
         re.Width = e.X - re.X;
         re.Height = e.Y - re.Y;
         this.Invalidate();
     }
 }

 private void form_Paint(object sender, PaintEventArgs e)
 {
     SolidBrush myBrush;
     myBrush = new SolidBrush(Color.PaleTurquoise);
     e.Graphics.FillRectangle(myBrush, re);
 }

 private void Form1_Click(object sender, EventArgs e)
 {
     re = new Rectangle(0, 0, 0, 0);
     this.Invalidate();
 }