Form1.cs

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 Calculate_Invoice
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

        }

        
        private void calculateTotal()
        {
            if(txt_subTotal.Text != "")
            {
                // get the subtotal amount from the subtotal text box

                decimal subTotal = Convert.ToDecimal(txt_subTotal.Text);

                // set the discountPercent variable based on the value of the subtotal variable

                decimal discountPercent = 0m; //the m indicate a decimal value
                if (subTotal >= 500)
                {
                    discountPercent = .2m;
                }
                else if (subTotal >= 250 && subTotal < 500)
                {
                    discountPercent = .15m;
                }
                else if (subTotal >= 100 && subTotal < 250)
                {
                    discountPercent = .1m;
                }

                // calculate and assign the values for the discountAmount and invoiceTotal variable
                decimal discountAmount = subTotal * discountPercent;
                decimal invoiceTotal = subTotal - discountAmount;

                // format the values and display them in their text boxes 
                txtDiscountPercent.Text = discountPercent.ToString("p1"); // p1 = percent format with 1 decimal place
                txtDiscountAmount.Text = discountAmount.ToString("c"); // c = currenct format 
                txtTotal.Text = invoiceTotal.ToString("c");

                // move the focus to the subTotal text box 
                txt_subTotal.Focus();
            }
            else
            {
                MessageBox.Show("Sub Total Can Empty Field");
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
          
        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void label2_Click(object sender, EventArgs e)
        {

        }


        private void txt_subTotal_TextChanged(object sender, EventArgs e)
        {

        }

        private void btn_exit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            if(e.KeyData == Keys.Escape)
            {
                this.Close();
            }
            else if (e.KeyData == Keys.Enter)
            {
                calculateTotal();
            }
        }

        private void btn_cal_Click(object sender, EventArgs e)
        {
            calculateTotal();
        }

        private void txtNumberOnly(object sender, KeyPressEventArgs e)
        {
            if(!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar)){
                e.Handled = true;
                MessageBox.Show("Sub Total Can Be Char");
            }
        }
    }
}