public partial class Form1 : Form
    {
        private Point mouse_offset;
        public Form1()
        {
            InitializeComponent();
        }
        private void label1_MouseUp(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                Point mousePos = Control.MousePosition;
                mousePos.Offset(mouse_offset.X, mouse_offset.Y);
                ((Control)sender).Location = ((Control)sender).Parent.PointToClient(mousePos);
            }
        }

        private void label1_MouseDown(object sender, MouseEventArgs e)
        {
            mouse_offset = new Point(-e.X, -e.Y);
        }
    }