Request.QueryString["ID"] = "A";
string key = Request.QueryString["ID"];
myDropDownList.Items.FindByText("key").selected = true;或者
myDropDownList.Items.FindByValue("key").selected = true;
string key = Request.QueryString["ID"];
myDropDownList.Items.FindByText("key").selected = true;或者
myDropDownList.Items.FindByValue("key").selected = true;
ListItem li=ddl.Items.FindByText(Request["ID"].Tostring());
if (li!=null) //防止错误
li.Selected=true;