<asp:DropDownList runat="server" ID="ddlIdSelector"
     AppendDataBoundItems="True"
     EnableViewState="true"
     AutoPostBack="false"
    ItemType="city.IdParams" DataTextField="cId" DataValueField="cId"/>
  <asp:Button runat="server" ID="btnSave" Text="Save" OnClick="btnSave_OnClick"/>


protected void Page_Load(object sender, EventArgs e)
    {
       if (!IsPostBack)
        {
            ddlIdSelector.DataSource = ids.Values;
            ddlIdSelector.DataBind();
        }
    }

    protected void btnSave_OnClick(object sender, EventArgs e)
    {
// ddlIdSelector.SelectedValue будет равно null
       string idKey = ddlIdSelector.SelectedValue;
     }