namespace MyDemo
{
  public class UserService : IUserService
    {
        public string IsUserExists(string userName)
        {
            try
            {
                  if(isUserExists)
                    return “#VALID#”.
                  else
                    return “#INVALID#”.;
            }
            catch (Exception exception)
            {
                throw new WebException(exception.Message);
            }
        }
    }
}