auto genrate column in gridview. when selectindexchange in dropdown in gridview.

See you can prefer
Put a textbox and your save button below your dropdownlist and show that on your dropdwonselected indexchagned event.
You can show after getting row index
and you will get that row index in your rowcommand or

protected void drp_SelectedIndexChanged(object sender, EventArgs e)
    {
GridViewRow row = (GridViewRow)((DropDownlist)sender).NamingContainer;
Textbox txt=(TextBox)row.FindControl("TxtID");
txt.Visible=true;
}

Try on this way