private void pictureBox_MouseDown(object sender, MouseEventArgs e)
{
    start = true;
    startPoint = e.Location;
}
 
private void pictureBox_MouseMove(object sender, MouseEventArgs e)
{
    if (start) //if the mouse is pressed
    {
        //set rectangle size&location
        rect.X = startPoint.X;
        rect.Y = startPoint.Y;
        rect.Width = e.X - startPoint.X;
        rect.Height = e.Y - startPoint.Y;
 
        //force the form to redrawn
        this.Invalidate();
    }
}
 
private void pictureBox_Paint(object sender, PaintEventArgs e)
{
    graph = e.Graphics;
    graph.DrawRectangle(Pens.Red, rect.Top, rect.Left, rect.Width, rect.Height);
}