How to populate a dropdown list with another ddl

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C#

how do i populate a dropdownlist with another dropdownlist list using a selected value, i tried the following code but it populates the ddl with business logic.

protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
     {
            Service obj = new Service ();
            List <Service > slist = new List<Service> ();
            Int32 ser = Convert .ToInt32  (DropDownList2.SelectedValue);
            slist = obj.allServcieBYfk(ser);
            drpDwnService.DataSource = slist;
            
            drpDwnService.DataBind();
        }