Добавьте опцию "Все" в список и измените ее динамически на основе выбора в раскрывающемся списке в разделе ASP.NET

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I have a drop down list with names of countries and a list box with different types of season like summer, winter etc In ASP.Net and both of them gets updated from a table in my database in the page_load event. During the Page_load event my drop down list and list box gets updated with all the available countries and seasons present in the table in the database. My list box changes dynamically after the country is selected in my drop down list. For example, if I choose USA, then season would change dynamically like fall, winter, summer. Right now it works perfectly. The only thing I want is to add an "ALL" option at the top of my list box if a country is selected which has all the seasons. For example if I choose country "X" in my drop down list and it has all the weathers present in my database, the list box would have an "ALL" option at the top. Basically the "ALL" option would pop up only when all the seasons during the page_load event matches the seasons after a country is selected.

Что я уже пробовал:

Я попытался получить общее количество списка после того, как моя страница загрузилась в первый раз, как это

protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                 
                int countListBox = lb.Items.Count;  
            }

        }

protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
        {
                       
            if (lb.Items.Count == countListBox)
            {
                lb.Items.Insert(0, "All");
            }
        }

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