TodoConsoleApplication

using System;
using System.IO;

namespace TodoApp
{
    class Program
    {
        static void ColoredText(string text,ConsoleColor color)
        {
            Console.ForegroundColor  = color;
            Console.WriteLine(text);
            Console.ResetColor();
        }
        static void ViewTODO(string[] lines)
        {
            Console.WriteLine("Things you have to do:");
            int i = 1;
            while (i < lines.Length) {
                Console.WriteLine($"\t{i}-{lines[i]}");
                i++;
            }
        }
        static void RemoveAt<T>(ref T[] arr, int index)
        {
            for (int a = index; a < arr.Length - 1; a++)
            {
                // moving elements downwards, to fill the gap at [index]
                arr[a] = arr[a + 1];
            }
            // finally, let's decrement Array's size by one
            Array.Resize(ref arr, arr.Length - 1);
        }
        static void Main()
        {
            // Path for the data.txt file.
            string path = @"data.txt";
            string firstName,input;
            Console.SetWindowSize(Console.LargestWindowWidth/2, Console.LargestWindowHeight/2);
            // Checks if the user is using this program for the first time.
            if (!File.Exists(path))
            {
                // Greetings.
                ColoredText("Welcome to TODO-listu.", ConsoleColor.Cyan);
                // Creates the data.txt file.
                ColoredText("Apparently this is your first time using the software." +
                    "\nCan you please provide us with your name ?", ConsoleColor.Cyan);
                firstName = Console.ReadLine();
                using (StreamWriter sw = File.CreateText(path))
                {
                    sw.WriteLine($"{firstName}");
                }
                string[] lines = System.IO.File.ReadAllLines(path);
            }
            // The user did use the program before.
            else
            {
                string[] lines = System.IO.File.ReadAllLines(path);
                firstName = lines[0];
                ColoredText($"Hey {firstName}, We're happy to have you back!", ConsoleColor.Cyan);
            }
            while(true)
            {
                if (File.ReadAllLines(path).Length == 1)
                {
                    Console.WriteLine("Your TODO list is empty.");
                }
                else
                {
                    ViewTODO(File.ReadAllLines(path));
                }
                ColoredText("Please pick a command:", ConsoleColor.Cyan);
                ColoredText("add: Add mode, to add multiple elements to the list.\ndel: Delete an element from the list." +
                    "\nclear: This command deletes the whole list.\nesc: To exit the program.", ConsoleColor.Magenta);
                input = Console.ReadLine();
                // Add mode.
                if (input == "add")
                {
                    Console.Clear();
                    while (true)
                    {
                        ColoredText("Add mode activated, Type 'esc' to go back to main menu:", ConsoleColor.Cyan);
                        input = Console.ReadLine();
                        if (input == "esc")
                        {
                            Console.Clear();
                            break;
                        }
                        else if (input == "")
                        {
                            Console.Clear();
                            ColoredText("You can't add an empty element.", ConsoleColor.Red);
                        }
                        else {
                            using (System.IO.StreamWriter file = new System.IO.StreamWriter(path, true))
                            {
                                file.WriteLine($"{input}");
                            }
                            Console.Clear();
                            ColoredText($"The element '{input}' has been added to the list succsefully.", ConsoleColor.Green);
                        }
                    }
                }
                // Delete mode.
                else if (input == "del")
                {
                    string[] lines = File.ReadAllLines(path);
                    ColoredText("Please tell us the index of the element you want to delete ?:", ConsoleColor.Cyan);
                    input = Console.ReadLine();
                    if ((Int32.Parse(input) == 0) || (Int32.Parse(input) > lines.Length-1))
                    {
                        Console.Clear();
                        ColoredText($"There is no element with the index {input}.", ConsoleColor.Red);
                        continue;
                    }
                    else
                    {
                        Console.Clear();
                        ColoredText($"The element '{lines[Int32.Parse(input)]}' has been deleted succsefully.", ConsoleColor.Green);
                        RemoveAt(ref lines, Int32.Parse(input));
                        System.IO.File.WriteAllLines(@"data.txt", lines);
                    }

                }
                // Clear mode.
                else if (input == "clear")
                {
                    Console.Clear();
                    ColoredText($"The TODO list has been cleared succsefully.", ConsoleColor.Green);
                    using (StreamWriter sw = File.CreateText(path))
                    {
                        sw.WriteLine(firstName);
                    }
                }
                // Exit the program
                else if (input == "esc")
                {
                    Console.Clear();
                    break;
                }
                // Do nothing if unvalid input.
                else
                {
                    Console.Clear();
                    ColoredText($"The command '{input}' doesn't exist.", ConsoleColor.Red);
                    continue;
                }
            }

        }
    }
}