kurec C# v2.0 - ft to inch

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

namespace up3cc
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        double[,] k = new double[4, 4];


        private void Form1_Load(object sender, EventArgs e)
        {


            comboBox1.Items.Add("ft");
            comboBox1.Items.Add("inch");
            comboBox1.Items.Add("miles");
            comboBox1.Items.Add("yards");
            comboBox1.SelectedIndex = 0;
            comboBox2.Items.Add("ft");
            comboBox2.Items.Add("inch");
            comboBox2.Items.Add("miles");
            comboBox2.Items.Add("yards");
            comboBox2.SelectedIndex = 0;
            k[0, 0] = 1;
            k[0, 1] = 2;
            k[0, 2] = 3;
            k[0, 3] = 4;
            k[1, 0] = 5;
            k[1, 1] = 6;
            k[1, 2] = 7;
            k[1, 3] = 8;
            k[2, 0] = 9;
            k[2, 1] = 10;
            k[2, 2] = 11;
            k[2, 3] = 12;
            k[3, 0] = 13;
            k[3, 1] = 14;
            k[3, 2] = 15;
            k[3, 3] = 16;
        }

        void calc()
        { textBox4.Text = (double.Parse(textBox1.Text) * k[comboBox1.SelectedIndex, comboBox2.SelectedIndex]).ToString(); }
        private void button1_Click(object sender, EventArgs e)
        {
           textBox4.Text = (double.Parse(textBox1.Text) * k[comboBox1.SelectedIndex, comboBox2.SelectedIndex]).ToString();


        }

        private void button2_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            textBox1.Clear();
            textBox4.Clear();
        }
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
                calc();


        }


        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
                calc();
        }

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
                calc();
        }


    }
}