Depreciation Calculator

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 Depreciation_Calculator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            comboBoxLife.SelectedIndex = 0;
            radioButtonSLine.Checked = true;

        }

        private void buttonUpdate_Click(object sender, EventArgs e)
        {

            if (double.Parse(textBoxICost.Text) <= double.Parse(textBoxSVal.Text))
                MessageBox.Show("Initial Cost must be greater than Salvage Cost");
            else
            {

                double depreciation = 0, bookVal = 0;
                int life = int.Parse(comboBoxLife.GetItemText(comboBoxLife.SelectedItem));
                double asset = double.Parse(textBoxICost.Text);
                double sVal = double.Parse(textBoxSVal.Text);
                listBoxOutput.Items.Clear(); // clear ouput


                if (radioButtonSLine.Checked)
                {
                    depreciation = SLN(asset, sVal, life);
                    bookVal = asset;
                    listBoxOutput.Items.Add("YEAR\t\tDEPRECIATION\t\tBOOK VALUE");
                    for (int x = 0; x < life; ++x)
                    {
                        bookVal -= depreciation;

                        listBoxOutput.Items.Add(x + 1 + "\t\t$" + string.Format("{0:F2}", depreciation)  + "\t\t\t$" + string.Format("{0:F2}", bookVal) );
                    }
                }
                else
                {
                    int y = life;
                    bookVal = asset;
                    listBoxOutput.Items.Add("YEAR\t\tDEPRECIATION\t\tBOOK VALUE");
                    for (int x = 0; x < life; ++x)
                    {
                        depreciation = (asset - sVal) * (y / SYD(life));
                        bookVal = bookVal - depreciation;
                        listBoxOutput.Items.Add(x + 1 + "\t\t$" + string.Format("{0:F2}", depreciation) + "\t\t\t$" + string.Format("{0:F2}", bookVal) );
                        --y;
                    }

                }
            }

        }


        private double SLN(double asset, double sVal, int life)
        {
            double depreciation;
            return depreciation = (asset - sVal) / life;
        }

        private double SYD(int life)
        {
            double depreciation;
            return depreciation = life * (life+1) / 2;
        }

        private void buttonExit_Click(object sender, EventArgs e)
        {
            Close();
        }
    }
}