using static WebDriverTestsCSharpSix.CSharpSix.StaticUsingSyntax.TimestampBuilder;
using static WebDriverTestsCSharpSix.CSharpSix.StaticUsingSyntax.UniqueEmailGenerator;
//....
public User RegisterUser(string email = null, string password = null, string userName = null)
{
//..
    if (string.IsNullOrEmpty(email))
    {
        email = BuildUniqueEmailTimestamp();
    }
//..
    return user;
}