我要根据combobox1里面的内容设置combobox2里的内容 为什么if语句没有执行

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            comboBox2.Items.Clear();            
            if (comboBox1 .SelectedItem.ToString ()  == "学号")
            {
                comboBox2.Items.Add(">");
                comboBox2.Items.Add(">=");
                comboBox2.Items.Add("<");
                comboBox2.Items.Add("<=");
                comboBox2.Items.Add("=");
                comboBox2.Items.Add("<>");
            }
            else if (comboBox1.SelectedItem.ToString() == "姓名")
            {
                comboBox2.Items.Add("=");
                comboBox2.Items.Add("like");
            }
            else
            {
                comboBox2.Items.Add("=");
            }
        }