小编pro*_*r21的帖子

我想在数据库中保存checkboxlist数据.我在SQL中创建了一个名为'tblSubject'的表,并在web.config中创建了connectionstring.但是我仍然得到了erorr:

未设置为对象实例的用户代码对象引用未处理NullReferenceException .

这是c#中的代码:

 private void PopulateSubjects()
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select * from subjects";
            cmd.Connection = conn;
            conn.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    ListItem item = new ListItem();
                    item.Text = sdr["Subject"].ToString();
                    item.Value = sdr["Id"].ToString();
                    item.Selected = Convert.ToBoolean(sdr["IsSelected"]);
                    chbox.Items.Add(item);
                }
            }
            conn.Close();
        }
    }
}

protected void Button1_Click(object sender, EventArgs e)
{
    using (SqlConnection conn = new SqlConnection())
    { …