DropDownList inside Gridview selectedindexchanged not firing on default selected item during dropdown bind

Question

I have a Gridview with dropdownlist is created dynamically in OnRowDataBound event of gridview, initially I am setting a selected value.

The problem is when I switch to different index of dropdown its working fine but when I change to the default selected index the SelectedIndexChanged is not fired.

Kindly help me..

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
    {
 DropDownList DropDownList1 = new DropDownList();
                    DropDownList1.ID = "DropDownList1";
                    DropDownList1.SelectedIndexChanged += new EventHandler(DropDownList1_SelectedIndexChanged);
                    DropDownList1.EnableViewState = true;
                    DropDownList1.AutoPostBack = true;
                    DropDownList1.EnableViewState = true;
                    string sql1 = ".....";
                    DataTable dtDDL = new DataTable();
                    dtDDL = SQL.ReturnDataTable(sql1);
                    if (dtDDL.Rows.Count > 0)
                    {
                        DropDownList1.DataSource = dtDDL;
                        DropDownList1.DataTextField = "CODE";
                        DropDownList1.DataValueField = "CODE";
                        DropDownList1.DataBind();
                        DropDownList1.Font.Size = 8;
                        //DropDownList1.Items.Insert(0, new ListItem("0", "0"));
                    }

                        DropDownList1.SelectedValue = dtShift.Rows[0]["SHIFT_CODE"].ToString();
                        DropDownList1.ToolTip = dtShift.Rows[0]["ShiftTime"].ToString();
                  }






  protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
       //not coming here for default index changed
    }