dropdown SelectedIndexChanged event is not triggering if index is changed in page_load event

Question

I have a drop down & a label, drop down is binded with a dictionary. When user changes the selected value of drop down I want to update the label. Following code works well but i want to set the initial value of label, I set the value of selected index in page_load, however event does not trigger. How to fix it? is there any page event which can help me solve the issue. I know i can fix it using javascript but I do not want to use JS.

 public partial class WebForm1 : System.Web.UI.Page
        {
                Dictionary<string, string> myDictionary = new Dictionary<string, string>();

            protected void Page_Load(object sender, EventArgs e)
            {
                if (!this.IsPostBack)
                {
                    myDictionary.Add("1", "Test Address 1");
                    myDictionary.Add("2", "Test Address 2");
                    myDictionary.Add("3", "Test Address 3");
                    myDictionary.Add("4", "Test Address 4");
                    myDictionary.Add("5", "Test Address 5");

                    drpTest.DataSource = myDictionary;
                    drpTest.DataTextField = "Key";
                    drpTest.DataValueField = "Value";
                    drpTest.DataBind();

                    // I want to set the index & update the label lblAddress
                    drpTest.SelectedIndex = 2;


                }
            }

            protected void drpTest_SelectedIndexChanged(object sender, EventArgs e)
            {
                lblAddress.Text = drpTest.SelectedItem.Value;
            }

Answer 1

change label text on pageload. See below.

protected void Page_Load(object sender, EventArgs e)
{
     if (!this.IsPostBack)
     {
          myDictionary.Add("1", "Test Address 1");
          myDictionary.Add("2", "Test Address 2");
          myDictionary.Add("3", "Test Address 3");
          myDictionary.Add("4", "Test Address 4");
          myDictionary.Add("5", "Test Address 5");

          drpTest.DataSource = myDictionary;
          drpTest.DataTextField = "Key";
          drpTest.DataValueField = "Value";
          drpTest.DataBind();

          drpTest.SelectedIndex = 2;
          lblAddress.Text = drpTest.SelectedItem.Value;     **// add this**
     }
}

Hope it works for you.

Answer 2

you should call this function at page load

drpTest_SelectedIndexChanged(null, null)

and it doesn't trigger as normal because you didn't change the dropdown selected value after initializing the page and ready to be used for client

Answer 3

当您定义下拉列表时，请包括drpTest.AutoPostBack = true这将在更改后触发SelectedIndexChanged事件。