using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if ((Control.ModifierKeys == (Keys.Control | Keys.Shift))&&(MouseButtons == MouseButtons.Left))
            {
                textBox1.Text = "I'm pressing CTRL + SHIFT + Mouse LEft button";
            }
        }
    }
}