获取GridView里的DropDownList的选出值

获取GridView里的DropDownList的选定值
选择DropDownList的选定值后会自动初始化，怎么办？
还有 DropDownList1_SelectedIndexChanged具体怎么写

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protected void ddlDe_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList ddl = (DropDownList)sender; //得到当前的DropDownList
GridViewRow gvr = (GridViewRow)ddl.NamingContainer; //获取对D……

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if(!IsPostBack)