How to make a C# phone book?

Question

C++ / C#

The user enters his number into the console. The program writes it to a txt file. If the file already has this number, then the program should generate an error. If there are less than 12 characters in the number, then the program should give an error

Answer 1

You can do something like this. You need to include the System.IO namespace.

string input = Console.ReadLine().Trin();
if (input.Length < 12) {
   Console.WriteLine("Ошибка!"); // Можно заменить на throw new Exception("Текст ошибки");
} else {
   string numbers = File.ReadAllLines(@"путь_к_файлу.txt");
   if (numbers.Contains(input)) {
      Console.WriteLine("Номер уже существует!");
   } else {
      File.AppendAllText(@"путь_к_файлу.txt",  input + Environment.NewLine);
   }
}