如果使用C#选择了dropdownlist值，则显示div

如果在下拉列表中选择了特殊的选项"，我正在尝试使Div出现.无论我尝试什么，我都无法使其正常工作.我在这里尝试了其他页面，但似乎没有任何作用.

I'm trying to make a Div appear if special "option" is selected in a Drop Down list. No matter what I try I can't make it work. I tried other pages here but nothing seems to make it work.

我的C#代码

protected void ddlSubject_SelectedIndexChanged(object sender, EventArgs e)
{
    if (ddlEmne.SelectedValue == "Lej os")
    {
        divselected.Visible = true;
    }
    if (ddlEmne.SelectedValue == "")
    {
        divselected.Visible = false;
    }
}

我的DropDownList:

My DropDownList:

<asp:DropDownList CssClass="margtop"    runat="server" OnSelectedIndexChanged="ddlSubject_SelectedIndexChanged">
                <asp:ListItem Value="Pakke Løsninger">Pakke Løsninger</asp:ListItem>
                <asp:ListItem Value="Spørgsmål">Spørgsmål</asp:ListItem>
                <asp:ListItem Value="Lej os">Lej os</asp:ListItem>
                <asp:ListItem Value="Andet">Andet</asp:ListItem>
            </asp:DropDownList>

和我要显示/隐藏的div:

and the div I want to show/hide :

<div   runat="server" style="visibility: hidden;">
    s
</div>

希望有人可以说我做错了.

Hope that someone can say that I'm doing wrong.