SelectedIndexChanged事件,真急!

Question

<asp:DropDownList id="DropDownList3" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList3_SelectedIndexChanged">
        <asp:ListItem Value="1">基础建筑</asp:ListItem>
        <asp:ListItem Value="2">特殊建筑</asp:ListItem>
        <asp:ListItem Value="3">军事建筑</asp:ListItem>
    </asp:DropDownList>
        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" /></div>protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
        {
            //
        }        protected void Button1_Click(object sender, EventArgs e)
        {
            DropDownList3.Items[0].Selected = true;
        }现在的问题是当我选择DropDownList3后,再点击Button1_Click事件,就会报错,提示"不能绑定多个选项"
请各位帮我看看是什么原因?

Answer 1

protected void Button1_Click(object sender, EventArgs e)
{
    for(int i=0;i<DropDownList3.Items.Count;i++)
    {
        DropDownList3.Items[i].Selected = false; 
    }
    DropDownList3.Items[0].Selected = true;
}

Answer 2

DropDownList3.SelectedItem = 0;

Answer 3

protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
        {
            //
        }        protected void Button1_Click(object sender, EventArgs e)
        {
            DropDownList3.ClearSelection();    //首先要取消原来的选择
            DropDownList3.Items[0].Selected = true;
        }
}