value is save in database but is not showing in dropdownlist

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VB

SQL

need some assistance. I am currently learning vb.net with sql.

I have two textboxes (txtValue and txtText) , a dropdownlist (ddl1) and save button (btnAdd). I insert value to txtValue and txtText (eg: txtValue: A and txtText: Apple). when i click button Add, The value will display in dropdownlist (like this; A - Apple) and both data will be save in the sqlserver (db).

im having issue where when i click button Add, the data is not display in the dropdownlist. But when i refresh my browser, the data will display in dropdownlist.

ASP.net

<asp:DropDownList ID="ddl1" runat="server" CssClass="form-control" AutoPostBack="true"></asp:DropDownList>

     <label class="form-control-static">Value</label>
     <asp:TextBox ID="txtValue" runat="server" CssClass="form-control"></asp:TextBox>
      <label class="form-control-static">Text</label>
      <asp:TextBox ID="txtText" runat="server" CssClass="form-control"></asp:TextBox>

      <button id="btnAdd" runat="server" style="min-width: 80px;">Add</button>

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load

        If Page.IsPostBack = False Then

            Dim rs As DataSet


            ddl1.Items.Clear()
            ddl1.Items.Add("")
            rs = db.ExecuteSelect("SELECT value, text FROM dropdownlist ", Session("CnnStr").ToString)
            For i = 0 To rs.Tables(0).Rows.Count - 1

                ddl1.Items.Add(rs.Tables(0).Rows(i).Item("value").ToString & " - " & rs.Tables(0).Rows(i).Item("text").ToString)

            Next
        End If
    End Sub



    Protected Sub btnAdd_ServerClick(sender As Object, e As EventArgs) Handles btnAdd.ServerClick
        db.ExecuteInsert("INSERT INTO dropdownlist (value, text ) VALUES( " & _
                             "'" & db.sqlstr(txtValue.Text) & "'," & _
                            "'" & db.sqlstr(txtText.Text) & "') ", _
                             Session("CnnStr").ToString)

    End Sub