Asp.net 下拉列表和更新面板

我开发了地址控制，它包含2个下拉列表（用于城市和国家）和几个文本框。第二个DropDownList数据源依赖于第一个DropDownList数据源

<fieldset>
 <legend><%=Title%></legend>
  <asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <div>
            <label for="<%=ddlCountry.ClientID %>">Country</label>
            <asp:DropDownList runat="server" ID="ddlCountry" 
              DataTextField="Name" DataValueField="Id" 
              DataSource="<%#Facade.Addresses.GetCountries() %>"
              AutoPostBack="true" 
              OnSelectedIndexChanged="ddlCountry_SelectedIndexChanged"
            />
        </div>
        <div>
            <label for="<%=ddlCity.ClientID %>">City</label>
            <asp:DropDownList runat="server" ID="ddlCity" 
                DataTextField="Name"   DataValueField="Name" />
        </div>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlCountry" EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>
<div>
    <label for="<%=txtStreet.ClientID %>">Street</label>
    <uc:TextBox ID="txtStreet" Text="<%#Address.Street %>" runat="server" />
</div>
<div>
    <label for="<%=txtBlock.ClientID %>">Block</label>
    <uc:TextBox ID="txtBlock" Text="<%#Address.Block %>" runat="server" />
</div>
<div>
</fieldset> 

它工作得很好。但如果页面上的其他控件导致回发，则当ddlCity中的SelectedValue设置为第一个（默认）值时

---

如何避免它？

将

页面上的代码移动到
页面加载
并将其放入
中！iPostBack

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ddlCountry.DataBind();
        ddlCity.DataSource = Facade.Addresses.GetCities(countryId);
        ddlCity.DataBind();
    }
}

放置ddlCountry.DataBind（）；内部if条件
为什么要检查两次“IsPostBack”？Fahad有解决方案。naveen的建议只是为了符合标准，尽管您只需要打开！IsPostback
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        ddlCountry.DataBind();
        ddlCity.DataSource = Facade.Addresses.GetCities(countryId);
        ddlCity.DataBind();
    }
}