用C#winform做三层架构 然后运行抛出了异样 求教如何改

public partial class User //用于验证用户是否合法以及更改用户登陆密码
    {
        public User() { }
        public bool IsLogin(string id, string password, ref string name)
        {
            string sql = string.Format("select * from [tbUser] where UserID='{0}' and UserPassWord='{1}'",id,password);
            using (SqlDataReader sdr = DBHelper.ExecuteReader(sql))   
            {
                if (sdr.HasRows)
                {
                    sdr.Read();
                    name = sdr["UserName"].ToString();
                    sdr.Close();
                    return true;
                }
                else
                {
                    return false;
                }
            }

黄色字体是抛出异常的提示 提示为     “DAL.DBHelper”的类型初始值设定项引发异常。