ComboBox绑定数据源时触发SelectedIndexChanged事件的处理办法

ComboBox最经常使用的事件就是SelectedIndexChanged。但在将ComboBox绑定到某个数据源的过程中，会触发SelectedIndexChanged
事件，而这个时候用户并没有选择内容，其SelectedValue也不是对应字段的值。那么时写在SelectedIndexChanged中的处理代码就会因为SelectedValue的内容不正确引发异常。
一般网上找到的方法是添加一个标记位，在绑定前设置为false，绑定完成后设置回true。

二级选择框联动显示数据

public partial class frmProduct : Form
{
DataSet DS = new DataSet();
String ConnectionString = "integrated security=true; database=AdventureWorks;  server=localhost; ";
public frmProduct()
{
InitializeComponent();
}

private void frmProduct_Load(object sender, EventArgs e)
{
SqlDataAdapter da = new SqlDataAdapter("select ProductCategoryID,[Name] from Production.ProductCategory", ConnectionString)
;
cbbCategories.SelectedIndexChanged -= new EventHandler(cbbCategories_SelectedIndexChanged);
da.Fill(DS, "ProductCategory");
cbbCategories.DataSource = null;
cbbCategories.ValueMember = "ProductCategoryID";
cbbCategories.DataSource = DS.Tables["ProductCategory"];
cbbCategories.SelectedIndexChanged += new EventHandler(cbbCategories_SelectedIndexChanged);
cbbCategories.DisplayMember = "Name";//这句放在事件委托之后才会有联动效果，下同
}

private void cbbCategories_SelectedIndexChanged(object sender, EventArgs e)
{
SqlDataAdapter da = new SqlDataAdapter("select ProductSubCategoryID,[Name] from Production.ProductSubCategory where  ProductCategoryID=" + cbbCategories.SelectedValue.ToString(), ConnectionString)
;
if (DS.Tables["ProductSubCategory"] != null)
{
DS.Tables["ProductSubCategory"].Clear();
}
da.Fill(DS, "ProductSubCategory");
cbbSubCategories.SelectedIndexChanged -= new EventHandler(cbbSubCategories_SelectedIndexChanged);
cbbSubCategories.DataSource = null;
cbbSubCategories.ValueMember = "ProductSubCategoryID";
cbbSubCategories.DataSource = DS.Tables["ProductSubCategory"];
cbbSubCategories.SelectedIndexChanged += new EventHandler(cbbSubCategories_SelectedIndexChanged);
cbbSubCategories.DisplayMember = "Name";
}

private void cbbSubCategories_SelectedIndexChanged(object sender, EventArgs e)
{
if (cbbSubCategories.SelectedIndex == -1)
return;
SqlDataAdapter da=new SqlDataAdapter("select * from Production.Product where  ProductSubCategoryID=" + cbbSubCategories.SelectedValue.ToString(), ConnectionString);
dgvProduct.DataSource = null;
if (DS.Tables["Product"] != null)
DS.Tables["Product"].Clear();
da.Fill(DS, "Product");
dgvProduct.DataSource = DS.Tables["Product"];
}
}