C# TryParse TextBox Issue

See more:

C#

Hello,

I'm new here. In my code below, I'm getting an "exception" thrown with additional info saying "Input string was not in a correct format".

I understand what is going on (sort of) but I don't know how to resolve it. If there is a number in multiplyBx (a text box) but no numbers in either startBx or endBx (two other text boxes) I get the aforementioned error above. But I thought I fixed this problem using the TryParse in my code. In some tests the TryParse certainly works, preventing the same type of exception being thrown, if you have all the text boxes blank or with letters or with a number in either startBx or endBx and a letter or blank in multiplyBx. It's just when there is a number in multiplyBx and blank or letters in either startBx or endBx I get the error.

Can anyone advise how to resolve the problem please? Many thanks.

// ##Code Below##

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

        private void button1_Click(object sender, EventArgs e)
        {
            int outputvalue = 0;
            bool isNumber = false;

            isNumber = int.TryParse(startBx.Text, out outputvalue);
            isNumber = int.TryParse(endBx.Text, out outputvalue);
            isNumber = int.TryParse(multiplyBx.Text, out outputvalue);

            int answer = 0;
            listBox1.Items.Clear();
                        

            if (!isNumber)
            {
                MessageBox.Show("Please enter numbers in the text boxes.");
            }

            else
            {
                int loopStart = int.Parse(startBx.Text);
                int loopEnd = int.Parse(endBx.Text);
                int multiplyBy = int.Parse(multiplyBx.Text);

                for (int i = loopStart; i <= loopEnd; i++)
                {
                    answer = multiplyBy * i;
                    listBox1.Items.Add(i + " multiplied by " + multiplyBy + " = " + answer.ToString());


                    
                }
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
            startBx.Clear();
            endBx.Clear();
            multiplyBx.Clear();
        }
    }
}