DropDown在VB.Net中的SelectedIndexChange上显示0的选定值

Question

我有一个下拉列表，我使用JQuery，AJAX webservice绑定它们，但是当我试图在SelectedIndexChange上检索该DropDownList的选定值时。我得到0（零）

绑定DropDownList之前

<asp:DropDownList ID="ddlbillNumber" runat="server" OnSelectedIndexChanged="ddlbillNumber_SelectedIndexChange" 
                                    AutoPostBack="true" >
                                    <asp:ListItem Value="0" Text="--Select--"></asp:ListItem>
                                </asp:DropDownList>

JSON＆amp; AJAX代码：

function GetBillNumbers() {
            //debugger;
            var ClientName = $("#<%=HiddenFieldCompanyId.ClientID %>").val();
            $.ajax({
                type: "POST",
                url: "<%=mstr_WebsitePath%>webservice/ClientSearch.asmx/GetClientBillNumber",
                data: "{ 'ClientName' : '" + ClientName + "' }",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (r) {
                    var ddlbillNumber = $("[id*=ddlbillNumber]");
                    ddlbillNumber.empty().append('<option value="0">--select--</option>');
                    $.each(r.d, function () {
                        ddlbillNumber.append($("<option></option>").val(this['Value']).html(this['Text']));
                    });
                }
            });
        }

在浏览器代码中绑定DropDownList后：

<select name="ddlbillNumber" id="ctl00_body_ddlbillNumber">
<option value="0">--select--</option>
  <option value="VMPL/Delhi/001/15-16">VMPL/Delhi/001/15-16</option>
</select>

然后我选择第二个选项意味着VMPL / Delhi / 001 / 15-16但是获得0 代码：

Protected Sub ddlbillNumber_SelectedIndexChange(ByVal sender As Object, ByVal e As EventArgs)
Dim Bill As String = ddlbillNumber.SelectedItem.Value // Here I am getting zero
            If Bill <> "" Then
                Dim Vasundhara As String = Bill.Substring(0, 2)
                If Vasundhara = "VI" Then
                    GetInfotechBill(Bill, 1)
                ElseIf Vasundhara = "VM" Then
                    GetInfotechBill(Bill, 0)
                End If
                PanelPrintDuplicatBill.Visible = True
            End If
End Sub

附上我的屏幕截图