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

namespace Deluxe_Number_Guesser
{
    public partial class guessGameForm : Form
    {
        // Declare variables
        int numOfTries;
        int guessNumber;
        int answer;
        bool done = false;

        public guessGameForm()
        {
            InitializeComponent();
        }

        private void guessGameForm_Load(object sender, EventArgs e)
        {
            // Initialize variables
            numOfTries = 0;
            guessNumber = 0;

            // Generate randon number
            Random rand = new Random();

            // Initialize number generate, 1 - 20; ensures a new number is generated
            // each time the form is loaded
            answer = rand.Next(1, 20) + 1;

        }

        public void submitButton_Click(object sender, EventArgs e)
        {
            try
            {
                if (nxtGuessTxtBox.Text == "")
                {
                    MessageBox.Show("Number is required", "Entry Error");
                    nxtGuessTxtBox.Focus();
                }
                else
                {
                    guessNumber = Convert.ToInt32(nxtGuessTxtBox.Text);

                    if (guessNumber == 0 || guessNumber >= 21)
                    {
                        MessageBox.Show("Enter a number between 1 to 20", "Entry Error");
                        nxtGuessTxtBox.Focus();
                        nxtGuessTxtBox.SelectAll();
                    }
                    else
                        while (!done && numOfTries < 4)
                        {
                            if (guessNumber == answer)
                            {
                                messageLabel.Text = "You guessed correctly";
                                randNumberLabel.Text = "Play again";
                                numOfTries++;
                                done = true;
                                break;
                            }
                            else if (numOfTries >= 3)
                            {
                                messageLabel.Text = "You exceeded guess";
                                numOfTries++;
                                randNumberLabel.Text = "The number is " + answer;
                                nxtGuessTxtBox.Focus();
                                nxtGuessTxtBox.SelectAll();
                                done = true;
                                break;
                            }
                            else if (guessNumber > answer)
                            {
                                messageLabel.Text = "You guessed too high";
                                highGuessTxtBox.Text = guessNumber.ToString();
                                numOfTries++;
                                nxtGuessTxtBox.Focus();
                                nxtGuessTxtBox.SelectAll();
                                break;
                            }
                            else if (guessNumber < answer)
                            {
                                messageLabel.Text = "You guessed too low";
                                lowGuessTxtBox.Text = guessNumber.ToString();
                                numOfTries++;
                                nxtGuessTxtBox.Focus();
                                nxtGuessTxtBox.SelectAll();
                                break;
                            }
                            
                        }numOfGuessTxtBox.Text = numOfTries.ToString();
                }
            }

            catch (FormatException)
            {
                MessageBox.Show("Enter numeric value", "Entry Error");
                nxtGuessTxtBox.Focus();
                nxtGuessTxtBox.SelectAll();  
            }
            
        }

        private void newGameButton_Click(object sender, EventArgs e)
        {
            // Clear appropriate fields from form
            this.nxtGuessTxtBox.Text = "";
            this.highGuessTxtBox.Text = "";
            this.lowGuessTxtBox.Text = "";
            this.numOfGuessTxtBox.Text = "";
            this.messageLabel.Text = string.Empty;
            this.randNumberLabel.Text = string.Empty;
                
            // Reset the variable, answer to a random integer, 1 to 20
            // generate randon number
            Random rand = new Random();
            answer = rand.Next(1, 20) + 1;

            numOfTries = 0;
            guessNumber = 0;

            // Set focus to guess text box
            this.nxtGuessTxtBox.Focus();
        }
      
        private void exitButton_Click_1(object sender, EventArgs e)
        {
            // Terminate program

            this.Close();
        }
    }
}

if (guessNumber == 0 || guessNumber >= 21)    //Your code

if (guessNumber < 0 || guessNumber > 20)    //Better code :)