js 如何触发服务器控件DropDownList的SelectedIndexChanged事件

[qq_34292420]

----------html
<asp:DropDownList ID="ddlCategoryId" runat="server" CssClass="select2" AutoPostBack="True" OnSelectedIndexChanged="ddlCategoryId_SelectedIndexChanged"></asp:DropDownList> 

---------事件
protected void ddlCategoryId_SelectedIndexChanged(object sender, EventArgs e)
{
Response.Redirect(Utils.CombUrlTxt("list.aspx", "channel_id={0}&category_id={1}&keywords={2}&property={3}",
this.channel_id.ToString(), ddlCategoryId.SelectedValue, this.keywords, this.property));
}

现在我想用js改变ddlCategoryId的值后触发后台的ddlCategoryId_SelectedIndexChanged事件该怎么做？？？

[雷诺兹]

document.getElementById("id").value = xxx;
document.getElementById("id").selectedIndex=xxxxx