如何在更新面板中的javascript中获取下拉列表的值

Question

I'm binding ddlstate on selectedindechanged event of ddlCountry only. ddlState dropdown value checking in javascript if it is "-1" then generate alert. When I check the value in javascript, it shows me a blank value, "". When I use Postbacktrigger for ddlState , I can get the value with javascript, but the smoothness of the page using async is better than Postbacktrigger. That is why I use async trigger. My main problem is ddlState 's value is not getting in javascript when I use async trigger while I can get it using Postback trigger.

JavaScript validation:

function validateForm()
{
    var ddlCountry = document.getElementById('<%=ddlCountry.ClientID%>');
    var ddlState = document.getElementById('<%=ddlState.ClientID%>');
    if (ddlCountry .value == "-1")
    {
        alert("Country  should not be blank.");
        ddlCountry .focus();
        return false;
    }
    if (ddlState .value == "-1")
    {
        alert("State should not be blank.");
        ddlState .focus();
        return false;
    }

    return true;
}

ASPX code:

<asp:DropDownList ID="ddlAcqModalityList" runat="server" CssClass="csstextbox" Width="207px" AutoPostBack="true" OnSelectedIndexChanged="ddlAcqModalityList_SelectedIndexChanged">
</asp:DropDownList>
<asp:UpdatePanel ID="updatePanelState" runat="server">
    <ContentTemplate>
        <asp:DropDownList ID="ddlState " runat="server" CssClass="csstextbox" Width="177px">
        </asp:DropDownList>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlCountry" EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>
<asp:Button ID="btnSave" runat="server" Width="80px" OnClientClick="return validateForm();" Text="Save" CssClass="cssbutton" OnClick="btnSave_Click" />

Code Behind:

protected void Page_Load(object sender, EventArgs e)
{
    if(!IsPostBack)
    {
        BindCountry()
        {
            strSQL = @"SELECT Country_ID,Country_Desc
                       FROM Country_Master";

            DataTable dataTableState = null;
            dataTableState = objSqlDbComm.ExecuteDatasetQuery(strSQL).Tables[0];

            var dictioneryCountry = new Dictionary<int, string>();
            foreach(DataRow dr in dataTableStudy.Rows)
            {
                dictioneryCountry .Add(Convert.ToInt32(dr["Country_ID"]), dr["Country_Desc"].ToString());
            }

            ddlCountry.DataTextField = "Value";
            ddlCountry.DataValueField = "Key";
            ddlCountry.DataSource = dictioneryCountry;
            ddlCountry.DataBind();
            ddlCountry.Items.Insert(0, new ListItem("[Select]", "-1"));
            ddlCountry.Items[0].Selected = true;
        }
    }
}

protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
{
    int countryID = Convert.ToInt32(ddlCountry.SelectedItem.Value);

    ifcountryID == -1)
    {
        return;
    }

    strSQL = @"SELECT State_ID,State_Desc
               FROM State_Master
               WHERE countryID = '" + countryID + @"';

    DataTable dataTableState = null;
    dataTableState = objSqlDbComm.ExecuteDatasetQuery(strSQL).Tables[0];

    var dictioneryState = new Dictionary<int, string>();
    foreach(DataRow dr in dataTableStudy.Rows)
    {
        dictioneryState .Add(Convert.ToInt32(dr["State_ID"]), dr["State_Desc"].ToString());
    }

    ddlState.DataTextField = "Value";
    ddlState.DataValueField = "Key";
    ddlState.DataSource = dictioneryState;
    ddlState.DataBind();
    ddlState.Items.Insert(0, new ListItem("[Select]", "-1"));
    ddlState.Items[0].Selected = true;
}

Answer 1

First of all I think you cannot call "ddlCountry.value" directly, that id is for the server control! You need to use, to get the client id.

document.getElementById('ddlCountry').value;

Another thing if you want to fire event on javascript when you change the dropdownlist you can add onchange client event like this, this will call a javascript function without posting the page (Refresh):

<asp:DropDownList ID="ddlState " runat="server" 
CssClass="csstextbox" Width="177px" onchange="validateForm()">

Then you can remove the eventtrigger and the updatepanel if you want.

Hope it helps.

Answer 2

        //javascript code
var hidStateValue=document.getElementByID('<%=hidStateValue.ClientID%>');
        if (hidStateValue.value == "-1")
                    {
                        alert("State should not be blank.");                
                        return false;
                    }

        //aspx code
        <asp:DropDownList ID="ddlState" AutoPostBack="true"  runat="server" CssClass="csstextbox" Width="177px" onselectedindexchanged="ddlState_SelectedIndexChanged">                                                                </asp:DropDownList>
            <asp:HiddenField ID="hidStateValue" runat="server" />

        //Code behind

          protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
         {
            //code
            hidStateValue.Value = ddlState.SelectedItem.Value;
         }
        protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
            {       
                hidStateValue.Value="";
                hidStateValue.Value = ddlState.SelectedItem.Value;      
            }