“在调用'Fill'之前，尚未初始化选择命令属性

public partial class Address : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
   string query;

    protected void Page_Load(object sender, EventArgs e)
    {

           con.Open();
           query = "select [Addid] from Address";
           int i=1;
           SqlCommand cmd = new SqlCommand(query, con);
           SqlDataAdapter da = new SqlDataAdapter();
           DataSet ds = new DataSet();
           DataTable dtab = new DataTable();
          da.Fill(ds);
          txtaddid.Text = ds.Tables[0].Rows[0][0].ToString();
          if (ds.Tables[0].Rows.Count == 0)
          txtaddid.Text = "Add" + i;
       else 
              if (ds.Tables[0].Rows.Count > 0)
       {
            int id = ds.Tables[0].Rows.Count + 1;
             txtaddid.Text = "Add" + id;
        }
  con.Close();
       }

我想做的是在页面加载时从地址表到文本框中的 addid 列中检索数据