源代码如下：
private void InsertNetNodeForm_Load(object sender, EventArgs e)
        {
            //绑定大类下拉列表控件cb_bigclass数据
            DataTable tbBigClass = new DataTable();
            da.SelectCommand = new OleDbCommand("select id,name from bigclass", conn);
            da.Fill(tbBigClass);

            cb_bigclass.DataSource = tbBigClass;
            cb_bigclass.DisplayMember = "name";
            cb_bigclass.ValueMember = "id";
        }

        private void cb_bigclass_SelectedIndexChanged(object sender, EventArgs e)
        {
            DataTable tbSmallClass = new DataTable();
            if (cb_bigclass.SelectedIndex > -1)
            {
                da.SelectCommand = new OleDbCommand("select id,name from smallclass where daleiid=" + cb_bigclass.SelectedValue.ToString(), conn);
                try
                {
                    da.Fill(tbSmallClass);
                    cb_smallclass.DataSource = tbSmallClass;
                    cb_smallclass.DisplayMember = "name";
                    cb_smallclass.ValueMember = "id";
                }
                catch (Exception exc)
                {
                }
            }
        }

问题：在窗体第一次启动时，cb_bigclass数据绑定引发了SelectedIndexChanged事件，但在事件处理程序中取不到SelectedValue的值，求解决方法，要么在数据绑定时不引发SelectedIndexChanged事件，要么在数据绑定完后如何正确设置第二个combobox的值。谢谢。