This is working code
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
DropDownList1.DataTextField = "user_name";
DropDownList1.DataValueField = "user_id";
DropDownList1.DataSource = getData();// get the data into the list you can set it
DropDownList1.DataBind();
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByText("your default selected text"));
}
}