c# ASP.Net Dynamically populated DropDownList loses selected index on submit button

Question

I dynamically populate a dropdownlist of all 50 states from an ArrayList on PageLoad. When the user selects the SUBMIT button (btnSubmit_Click event), the SelectedIndex property of the dropdownlist control is always 0 despite what selection the user selects.

---

Added more code to help troubleshooting. Getting a -1 both from the session variable (bbb) and from the ddlState.selectedindex (bbb).

HTML code in form:

<asp:DropDownList ID="ddlState" runat="server" AutoPostBack="True" 
    onselectedindexchanged="ddlState_SelectedIndexChanged" >
</asp:DropDownList>

Code Behind:

protected void Page_Load(object sender, EventArgs e)
{
    //------------------------------------------------
    // Populates state dropdownlists
    //------------------------------------------------
    if (!IsPostBack)
    {
        GetAllStatesForDdl(ddlDLState);
        GetAllStatesForDdl(ddlOldState);
        GetStatesForDdl(ddlState);
    }
}

private void GetAllStatesForDdl(DropDownList ddlStateList)
{
    AppInputFormProcessor getStates = new AppInputFormProcessor();
    ArrayList States = new ArrayList();
    States = getStates.GetAllStates();
    ddlStateList.DataSource = States;
    ddlStateList.DataBind();
}

private void GetStatesForDdl(DropDownList ddlStateList)
{
    AppInputFormProcessor getStates = new AppInputFormProcessor();
    ArrayList States = new ArrayList();
    States = getStates.GetStates();
    ddlStateList.DataSource = States;
    ddlStateList.DataBind();
}

protected void btnSubmit_Click(object sender, EventArgs e)
{
    int aaa = ddlState.SelectedIndex;
    int bbb = Convert.ToInt32(Session["ddlState"]);
}

protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
{
    Session["ddlState"] = ddlState.SelectedIndex;
}

Answer 1

When I had trouble with ViewState (that is what l suspect in your case) l used this to restore data to a dynamically populated dropdown object

    public void Page_Load(object sender, EventArgs e){
            if (!IsPostBack)
            {
                Databind();
            }
            else {
                LoadAllViewStates();
            }
    }
    private void Databind()
        {
            DataTable questionnaireDT = null;
            DataTable questionsDT = null;
            DataTable indicatorDT = null;

            DataView tempView = QuestionnaireDS.Select(DataSourceSelectArguments.Empty) as DataView;
            questionnaireDT = tempView.Table;
            ViewState["QuestionnaireDL"] = questionnaireDT;
            QuestionnaireDL.DataSource = ViewState["QuestionnaireDL"];
            QuestionnaireDL.DataBind();

            tempView = QuestionDS.Select(DataSourceSelectArguments.Empty) as DataView;
            questionsDT = tempView.Table;
            ViewState["QuestionList"] = questionsDT;
            QuestionList.DataSource = ViewState["QuestionList"];
            QuestionList.DataBind();

            tempView = IndicatorDS.Select(DataSourceSelectArguments.Empty) as DataView;
            indicatorDT = tempView.Table;
            ViewState["IndicatorLst"] = indicatorDT;
            IndicatorLst.DataSource = ViewState["IndicatorLst"];
            IndicatorLst.DataBind();
        }

        private void LoadAllViewStates()
        {
            QuestionnaireDL.DataSource = ViewState["QuestionnaireDL"];
            QuestionnaireDL.DataBind();

            QuestionList.DataSource = ViewState["QuestionList"];
            QuestionList.DataBind();

            IndicatorLst.DataSource = ViewState["IndicatorLst"];
            IndicatorLst.DataBind();
        }

To restore the selected index, I passed the selectedIndex into a hidden field.

Hope this helps?

By the way, why pass in the DropDownList object as a parameter? Instead call a parameterless function and populate the DropDownList object within the function.

Also, ensure that ViewState isnt switched off.

Answer 2

In your example, you are trying to get the selected value from a session variable, but there's no code shown that actually sets anything in the session.

Even if you have some sort of async call that sets a session variable, this is a very dangerous practice: as soon as someone opens up a 2nd tab, you risk the chance of data corruption.

Answer 3

This should work for you. However, I am sort of confused why you are passing the dropdown to the function to get the states. Do you have multiple dropdowns to be filled? I think we need to see your html to be of more help.

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

 private void GetStatesForDdl(DropDownList ddlStateList)
 {
     AppInputFormProcessor getStates = new AppInputFormProcessor();
     ArrayList States = new ArrayList();
     States = getStates.GetStates();
     ddlStateList.DataSource = States;
     ddlStateList.DataBind();
 }