problem with dropdownlist in asp.net

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C#

ASP.NET

i have a project where i want that if a user selects a value in the dropdownlist .. it must be shown in the label but the onselectedindexchanged method is not firing the values but if i click on any button present the same page then the value is being shown in the label..

the desire output is that the onselectedindexchanged must fire the value on its own

here's my code

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{

    string sdr= dropdownlist1.selectedvalue;
    label1.text= "Hi"+sdr;

 }

any help is appreciated.

Thnk u