ajax:CascadingDropDown将selectedIndex设置为0

Question

我有两个ajax:CascadingDropDown，，一个是state，一个是县。它的工作方式就像用户更改状态一样，然后县将被重新填充新的数据。但是，问题是，一旦县有了新的数据，所选的索引就会保持不变。当用户更改状态时，如何将所选索引设置为0？谢谢!

<asp:DropDownList ID="Location_State" runat="server">
</asp:DropDownList>

<ajax:CascadingDropDown ID="StateCascading" runat="server" Category="State" 
                        TargetControlID="Location_State" 
                        ServiceMethod="BindStateDropdown" 
                        ServicePath="CountyService.asmx">
</ajax:CascadingDropDown>

<asp:DropDownList ID="Location_County" runat="server">
</asp:DropDownList>

<ajax:CascadingDropDown ID="CountyCascading" runat="server" Category="County" 
                        TargetControlID="Location_County"
                        ParentControlID="Location_State"
                        ServiceMethod="BindCountyDropdown" 
                        ServicePath="CountyService.asmx">
</ajax:CascadingDropDown>

下面是填充县列表的函数

[WebMethod]
public CascadingDropDownNameValue[] BindCountyDropdown(string knownCategoryValues, string category)
{
    StringDictionary statedetails = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
    List<CascadingDropDownNameValue> countydetails = new List<CascadingDropDownNameValue>();
    string tableName = "county";
    string sqlSQL = "";
    string sqlConnStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ToString();
    SqlConnection sqlConn = new SqlConnection(sqlConnStr);
    sqlConn.Open();
    sqlSQL = "SELECT [NAME], [CNTY_FIPS] FROM " + tableName + " [county] WHERE (\[STATE_FIPS] = @STATE_FIPS order by name";
    SqlCommand cmdSql = new SqlCommand(sqlSQL, sqlConn);
    ((SqlParameter)cmdSql.Parameters.Add("@STATE_FIPS", SqlDbType.VarChar)).Value = statedetails["State"];
    SqlDataReader rdr = cmdSql.ExecuteReader();
    while (rdr.Read())
    {
        countydetails.Add(new CascadingDropDownNameValue(rdr["NAME"].ToString(), rdr["CNTY_FIPS"].ToString()));
    }
    rdr.Close();
    sqlConn.Close();
    return countydetails.ToArray();
}

Answer 1

我自己想办法解决。一个简单而有效的解决方案。只需在0.1秒后将ddl的索引更改为0，只要ajax请求快于0.1秒！

$(function () {
        $("#MainContent_Location_State").change(function () {
            setTimeout(function () { document.getElementById('MainContent_Location_County').selectedIndex = 0; }, 100);
        });
});