public class User
{
    public string Username { get; set; }
    public string Password { get; set; }
}

public void ReceiveUserString(string source)
{
    string[] UserString = source.Split(',');

    var User = new User();
    foreach (string u in UserString)
    {
        User.Username = string[0];
        User.Password = string[1];
    }
}

string[] UsersFromString = source.Split('|');

foreach (string user in UsersFromString)
{
    var User = new User();
    string[] Fields = user .Split(',');

    User.Username = Fields[0];
    User.Password = Fields[1];
}