int index = this.DropDownList1.Items.IndexOf(this.DropDownList1.Items.FindByValue("b"));//1
------解决方案--------------------
正解
或
DropDownList2.Items.FindByText("b").Selected=true;
int index = DropDownList2.SelectedIndex;
或将value值直接为索引
DropDownList2.Items.FindByText("b").Value ------解决方案--------------------
ListItem listItem = this.DropDownList1.Items.FindByText("1");//1为text
int index = this.DropDownList1.Items.IndexOf(listItem);//你像要的