DataTable dt = EmployerSet.Tables["EmployeesTable"];
DataRow dr = dt.NewRow();
dr["FirstName"] = "--选择所有--";
dt.Rows.InsertAt(dr, 0);
this.comboBox1.DataSource = dt;
this.comboBox1.DisplayMember = "FirstName";
DataTable dt = EmployerSet.Tables["EmployeesTable"];
DataRow dr = dt.NewRow();
dr["FirstName"] = "--选择所有--";
dt.Rows.InsertAt(dr, 0);
this.comboBox1.DataSource = dt;
this.comboBox1.DisplayMember = "FirstName";