How to detect a left mouse click on a Winform titlebar

private const long WM_NCLBUTTONDOWN = 0xA1;//161
   private const long WM_NCLBUTTONUP = 0xA0;//160

protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_NCLBUTTONDOWN)
            {
                if (this.Opacity != 0.5)
                {
                    this.Opacity = 0.5;
                }
            }
            if (m.Msg == WM_NCLBUTTONUP)
            {
                if (this.Opacity != 1.0)
                {
                    this.Opacity = 1.0;
                }
            }
            base.WndProc(ref m);          
        }

using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private const long WM_NCLBUTTONDOWN = 0xA1;//161
        private const long WM_NCLBUTTONUP = 0xA0;//160

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_NCLBUTTONDOWN)
            {
                if (this.Opacity != 0.5)
                {
                    this.Opacity = 0.5;
                }
            }
            if (m.Msg == WM_NCLBUTTONUP)
            {
                if (this.Opacity != 1.0)
                {
                    this.Opacity = 1.0;
                }
            }
            base.WndProc(ref m);          
        }
    }
}