GridView not populating on Page_Load but on PostBack or Refresh - asp.net

Question

In an ASP.NET WebForms application there are just two controls in an aspx page, a DropDownList and a GridView. There is no default selected value of DropDownList on Page_Load. Changing the selection in DropDownList populates GridView accurately.
When the page is requested with a URL parameter such as .../View_Details.aspx?C_ID=123, the selected value in DropDownList changes but GridView does not populate for the first time but refreshing the page shows the records for given URL parameter.
ASPX markup:
<%# Page Title="Data" Language="vb" AutoEventWireup="false" MasterPageFile="~/HomePage.Master" CodeBehind="View_Details.aspx.vb" Inherits="App1.View_Details" %>
<asp:Content ID="Content4" ContentPlaceHolderID="BodyCP" runat="server">
<asp:DropDownList ID="CIDCombo" runat="server" DataSourceID="SqlDSCID" DataTextField="CName" DataValueField="CID" AutoPostBack="true"></asp:DropDownList>
<asp:SqlDataSource ID="SqlDSCID" runat="server" ... ></asp:SqlDataSource>
<asp:GridView ID="gvData" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:BoundField DataField="Fld1" />
<asp:BoundField DataField="Fld2" />
...
</Columns>
</asp:GridView>
</asp:Content>
Code Behind:
Private C_ID As Long
Dim con As SqlConnection = New SqlConnection(ConfigurationManager.Connect...)
Dim cmd As New SqlCommand()
Dim stSqlQry As String = ""
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
C_ID = CLng(Request.QueryString("C_ID"))
If IsPostBack Then
Else
If C_ID > 0 Then
CIDCombo.SelectedValue = C_ID.ToString
LoadGVData(C_ID)
End If
End If
End Sub
Private Sub CIDCombo_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles CIDCombo.SelectedIndexChanged
If CIDCombo.SelectedIndex >= 0 AndAlso CLng(CIDCombo.SelectedValue) > 0 Then
LoadGVData(CLng(CIDCombo.SelectedValue))
End If
End Sub
Private Sub LoadGVData(ByVal lnCID As Long)
Try
If con.State <> ConnectionState.Open Then con.Open()
Dim da As SqlDataAdapter = New SqlDataAdapter()
stSqlQry = "SELECT Fld1, Fld2 ... WHERE CID = #CID"
da = New SqlDataAdapter()
cmd = New SqlCommand(stSqlQry, con)
cmd.Parameters.AddWithValue("#CID", lnCID)
Dim dtDataTableInc As DataTable = New DataTable("t_Data")
da.SelectCommand = cmd
da.Fill(dtDataTableInc)
'SOME DATA MANIPULATION WITH DATATABLE'
'****************************************************************************'
'DEBUG MODE SHOWS DataTable HAS ROWS BUT DON'T SHOW UP FIRST TIME IN GRIDVIEW'
'****************************************************************************'
gvData.DataSource = dtDataTableInc
gvData.DataBind()
Catch ex As Exception
'EXCEPTION HANDLING
Finally
If con.State <> ConnectionState.Closed Then con.Close()
End Try
End Sub

Answer 1

I see you have AutoEventWireup="false" put it on true.
Just a general note:
When working with DropDownLists and using AutoPostBack=True
make use of an UpdatePanel since the User gets frustrated when he always see a white page flickering :)
if you use an UpdatePanel you use the Onload event to populate your data
and put UpdateMode=Conditional
Good luck and happy coding.