using System;

using System.Collections.Generic;

​

public class Robot

{

    public string Name { get; private set; }

​

    public Robot() => Reset();

​

    public void Reset() 

    {

        // Use extra variable to not let name generator to possibly generate the same name

        var newName = NameGenerator.Generate();

        NameGenerator.FreeUp(Name);

        Name = newName;

    }

}

​

public static class NameGenerator

{

    private const string AllowedCharacters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    private static readonly Random _random = new Random();

    private static HashSet<string> _namesInUse = new HashSet<string>();

​

    public static string Generate()

    {

        string name;

        do

        { 

            name = $"{GetRandomCharacter()}{GetRandomCharacter()}{GetThreeRandomDigits()}";

        }

        while (_namesInUse.Contains(name));

​

        _namesInUse.Add(name);

​

        return name;

    }

​

    public static void FreeUp(string name)

    {

        _namesInUse.Remove(name);

    }

​

    private static char GetRandomCharacter() => AllowedCharacters[_random.Next(0, AllowedCharacters.Length)];

​

    private static string GetThreeRandomDigits() => _random.Next(0, 1000).ToString("D3");

}