Listbox duplicates(repeats) on postback

Question

I understand that this question is already posted and an answer has been given, but my case is different as I am populating multiple dropdown on a single method and so if I follow this link After every postback dropdownlist items repeats it will only retain the items in the last dropdownlist as it would clear all the lists before it.

Below would be my code, I use this method as a filter for gridview.

private void BindDropDownList()
{
        PopulateDropDown(ddlgvsite, lblsite.Text);
        PopulateDropDown(ddlgvskill, lblskill.Text);
        PopulateDropDown(ddlgvshift, lblshift.Text);
        PopulateDropDown(ddlgvtm, lbltm.Text);
        PopulateDropDown(ddlgvgrpm, lblgrpm.Text);
        PopulateDropDown(ddlgvopsm, lblopsm.Text);
        PopulateDropDown(ddlgvltype, lblltype.Text);
}
private void PopulateDropDown(ListBox ddl, string columnName)
{
        ddl.DataSource = BindDropDown(columnName);
        ddl.DataTextField = columnName;
        ddl.DataValueField = columnName;
        ddl.DataBind();
        ddl.Items.Insert(0, new ListItem("Please Select", "0"));
}
private void setDropdownselectedItem(string selectedvalue, ListBox ddl)
{
        if (!string.IsNullOrEmpty(selectedvalue))
        {
            ddl.Items.FindByValue(selectedvalue).Selected = true;
        }
}
protected void DropDownChange(object sender, EventArgs e)
    {
        ListBox dropdown = (ListBox)sender;
        string selectedValue = dropdown.SelectedItem.Value;
        switch (dropdown.ID.ToLower())
        {
            case "ddlgvsite":
                ViewState["Site"] = selectedValue;
                break;
            case "ddlgvskill":
                ViewState["Skill"] = selectedValue;
                break;
            case "ddlgvshift":
                ViewState["Shift"] = selectedValue;
                break;
            case "ddlgvtm":
                ViewState["TM"] = selectedValue;
                break;
            case "ddlgvgrpm":
                ViewState["GrpM"] = selectedValue;
                break;
            case "ddlgvopsm":
                ViewState["OpsM"] = selectedValue;
                break;
            case "ddlgvltype":
                ViewState["Ltype"] = selectedValue;
                break;
        }
        this.BindGrid();
    }
private DataTable BindDropDown(string columnName)
    {
        String strConnString = System.Configuration.ConfigurationManager.ConnectionStrings["connStr"].ConnectionString;
        MySqlConnection con = new MySqlConnection(strConnString);
        MySqlCommand cmd = new MySqlCommand("SELECT DISTINCT (" + columnName + ") FROM approved WHERE " + columnName + " IS NOT NULL", con);
        MySqlDataAdapter sda = new MySqlDataAdapter(cmd);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        return dt;
    }

So is there a way for me to avoid getting the lists repeated on postback? Please let me know. Thanks in advance.

Answer 1

Sounds like DataBind isn't clearing the old values, so you could try:

ddl.Items.Clear();

at the start of PopulateDropDown