如何为查询中已填充的字典的第一个选项添加值？

Question

我正在将查询的字典结果集绑定到字典以填充下拉列表，但我想为下拉列表添加默认值

我尝试先添加该值，但它不起作用。我也尝试过最后添加它并反转它。

         <asp:DropDownList ID="ddl_Reason" runat="server"  OnInit="ddl_Reason_Init"></asp:DropDownList>           


    protected void ddl_Reason_Init(object sender, EventArgs e)
    {
        ddlreason = (DropDownList)sender;



        ddlreason.DataSource = sr.ReturnReason();
        ddlreason.DataTextField = "Value";
        ddlreason.DataValueField = "Key";
        ddlreason.DataBind();

        //ddlreason.Items.Insert(0, new ListItem("--Select Reason--","0"));


    }



               var ddlreasonpop = (from a in pc.tbl_Reason
                            select new { a.Reason, a.Id }).ToDictionary(a => a.Id, a => a.Reason);


        Dic_reason = ddlreasonpop;
        Dic_reason.Add(0, "--Select Reason--");