Question

I have a aspx Page where I am using AJAX. like

<asp:UpdatePanel runat="server" ID="upPanelDDLProgram">
  <ContentTemplate>
    <asp:DropDownList ID="DDLProgram" runat="server" Width="194px" Height="18px" OnSelectedIndexChanged="OnDDLProgramChanged" AutoPostBack="true">
    </asp:DropDownList>
  </ContentTemplate>
</asp:UpdatePanel> 

and my code behind is like

    protected void Page_Load(object sender, EventArgs e)
    {
        //if (!IsPostBack)
        //{
        //    BindProgramDDL();
        //}
        BindProgramDDL();
    }

    protected void BindProgramDDL()
    {
        List<CcProgramEntity> programEntities = FormSaleSubmit_BAO.GetAllPrograms();

        DDLProgram.DataSource = programEntities;
        DDLProgram.DataTextField = "Shortname";
        DDLProgram.DataValueField = "Id";
        DDLProgram.DataBind();
        string programCode = programEntities[DDLProgram.SelectedIndex].Code;
    }

    protected void OnDDLProgramChanged(object sender, EventArgs e)
    {
        List<CcProgramEntity> programEntities = FormSaleSubmit_BAO.GetAllPrograms();
        string programCode = programEntities[DDLProgram.SelectedIndex].Code;
    }

the If condition is the page load event, is commented out. If I toggle the comment part of the page load event, it works perfect in both cases. My question is why is this heppening?

Answer 1

If I am getting you correct …….

DropDown list has data even you are not binding it second time after post back……….its becasuse its server side control and each serverside control has its view state with it thats y its not removing data.

IsPostBack – it true when do the post back by using serverside control like dropdown, checkbox , textbox…………When you load page first time this property is false but in subsequent request to same page value of this property is true. you can check msdn document for more detail about it.