C#winform combobox数据

combobox已经绑定list了可以再为其添加一个值吗？比如“--请选择--”
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假设subject1.getSuject()返回的是一个DataSet

DataRow dr = subject1.getSuject().Tables[0].NewRow();

dr["SubjectName"] = "全部";

dr["SubjectNo"] = -1;

cobSubject1.getSuject().Tables[0].Rows.InsertAt(dr, 0);

默认让combobox选择一项
cobSubject1.SelectIndex = 0