Web Forms :: Fire Selectindexchanged Event Of Dynamic Droopdown In Gridview?

I added dropdownlist & textbox controls to gridview row dynamically on button click.

my dropdown Datavaluefield="Amount" & DataTextfield="Product" . when I select a product item in dropdownlist the corresding product item Amount should display in Textbox in gridview.

and dropdown selectchangedevent is not firing why?

I write like this:

protected void ddl1_SelectedIndexChanged(object sender, EventArgs e)
{
GridViewRow gridViewRow = (GridViewRow)(((Control)sender).NamingContainer);
DropDownList ddl1 = (DropDownList)gridViewRow.FindControl("ddl1");
TextBox itProductQuantity = (TextBox)gridViewRow.FindControl("itProductQuantity");
itProductQuantity.Text = ddl1.SelectedValue;
}