gridview里的dropdown

for (int i = 0; i < GV_PlateSamples.Rows.Count; i++)
{
if ((DropDownList)GV_PlateSamples.Rows[i].FindControl("ddl_failurereason")).SelectedValue = "pcr")
}

}
请问这样找为什么不对呢 而且提示点不出来 SelectedValue
那正确的方法是什么呢？   

(DropDownList)GV_PlateSamples.Rows[i].FindControl("ddl_failurereason")).Controls(0).SelectedValue = "pcr")