protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
    {
        string str = ConfigurationSettings.AppSettings["dm"];
        SqlConnection conn = new SqlConnection(str);
        conn.Open();        string strselect="select * from users where user_yonghu='"+TextBox1.Text+"'";
        SqlCommand comm = new SqlCommand(strselect, conn);
        SqlDataReader dr = comm.ExecuteReader();
        if (dr.Read())
        {
            CustomValidator1.IsValid = false;
        }
        else
        {
            CustomValidator1.IsValid = true;
        }
        conn.Close();
    }这段代码是什么意思?