当SelectedItemEvent被触发时，SelectedItem不会改变.净下拉)

我有一个ASP下拉菜单，声明如下:

<asp:DropDownList ID="DropDown1" 
    runat="server" 
    OnSelectedIndexChanged="DropDown1_SelectedIndexChanged"
    AutoPostBack="true">
</asp:DropDownList>

数据按如下方式填写:

DropDown1.DataSource = source;
DropDown1.DataTextField = "Key";
DropDown1.DataValueField = "Value";
DdlCat1.DataBind();

"source"是字典:

Dictionary<int, string> source = new Dictionary<int, string>();

selectedIndexChanged方法如下所示:

public void DropDown1_SelectedIndexChanged(object sender, EventArgs e)
{
    DropDownList temp = (DropDownList)sender;
    Console.WriteLine(temp.SelectedValue);
}

现在假设在下拉菜单中有item1, item2和item3。目前我选择了item1。现在的问题是，如果我点击item2, SelectIndexChanged方法被触发，但SelectedValue总是停留在"item1"。那么，我做错了什么?