to close a application

public partial class Form4 : Form

    {

        //Variable to hold the number of times user entered text "Hi"

        private int errorCount = 0;

        public Form4()

        {

            InitializeComponent();

        }

        private void textBox1_TextChanged(object sender, EventArgs e)

        {

            //Chech if user has entered "Hi"

            if (textBox1.Text == "Hi")

            {

                //If user eneterd "Hi" first time

                if (errorCount < 1)

                {

                    //display Error Message

                    MessageBox.Show("textBox1.Text = Hi", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

                    //Increment error count

                    errorCount++;

                }

                //If user has entered "Hi" more than one time

                else

                {

                    //Close the application

                    this.Close();

                }

            }

        }

    }