ln.skyscanner/crawl/CredentialsGenerator.cs

using System;
using System.Collections;
using System.Collections.Generic;
namespace ln.skyscanner.crawl
{
    public class CredentialsGenerator : IEnumerable<Credential>
    {
        HashSet<String> userNames = new HashSet<string>();
        HashSet<String> passWords = new HashSet<string>();

        public CredentialsGenerator()
        {
        }


        public CredentialsGenerator AddUserNames(IEnumerable<string> usernames)
        {
            foreach (String userName in usernames)
                AddUserName(userName);

            return this;
        }
        public CredentialsGenerator AddUserName(String username)
        {
            userNames.Add(username);
            return this;
        }

        public CredentialsGenerator AddPasswords(IEnumerable<string> passwords)
        {
            foreach (String pw in passwords)
                AddPassword(pw);
            return this;
        }

        public CredentialsGenerator AddPassword(String password)
        {
            passWords.Add(password);
            return this;
        }

        public IEnumerator<Credential> GetEnumerator()
        {
            foreach (String username in userNames)
                foreach (String pw in passWords)
                    yield return new Credential(username, pw);
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }

    public class Credential
    {
        public String Username { get; }
        public String Password { get; }

        public Credential(String username,String password)
        {
            Username = username;
            Password = password;
        }
    }

}