using System.Collections.Generic;
public bool IsValid(string login, string password)
Dictionary<string, string> users = new Dictionary<string, string>
{"adminka","qwerty1234"},
{"vasya123","vasya123456"},
{"olya@mail.ru","qwerty1234"}
if (users.ContainsKey(login))
if (users[login].Contains(password))
Console.WriteLine(IsValid("admin", "123123"));
Console.WriteLine(IsValid("vasya123", "vasya123456"));