Fill GridView based on DropDownList.SelectedValue and Button Click - asp.net

Question

struggling since morning. the scenario is as follow:
I have a ASP.NET web page having
two dropdownlist
two buttons
One GridView
Wants to fill gridview based on dropdownlist selected value
here is the .aspx code
<%# Page Title="" Language="VB" MasterPageFile="~/Admin/ADMIN.master" AutoEventWireup="false" CodeFile="EditSTUdetail.aspx.vb" Inherits="Admin_EditSTUdetail" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
<style type="text/css">
.auto-style1
{
height: 42px;
}
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
<div align="center">
<fieldset>
<legend style="font-size: 16px;">SEARCH / EDIT / DELETE STUDENT RECORDS
</legend>
<div style="overflow: auto">
<div>
<asp:Label ID="lblMsg" runat="server" CssClass="lblresponse" />
<table style="margin: 3px auto 1px auto; height: 72px;">
<tr>
<td style="text-align: right;" class="auto-style2">Select Session/सत्र का चयन करें :
</td>
<td class="auto-style3">
<asp:DropDownList ID="ddlSession" runat="server" AppendDataBoundItems="True" Width="236px" Height="28px">
<asp:ListItem Text="--Select Session--" Value=""></asp:ListItem>
<asp:ListItem Text="" Value=""></asp:ListItem>
</asp:DropDownList>
</td>
<td>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="*" ControlToValidate="ddlSession" ForeColor="#990000"></asp:RequiredFieldValidator></td>
</tr>
<tr>
<td style="text-align: right;" class="auto-style1">Select Course /कक्षा का चयन करें :
</td>
<td>
<asp:DropDownList ID="ddlCourse" runat="server" AppendDataBoundItems="true" Width="236px" Height="28px">
<asp:ListItem Text="--Select Course--" Value=""></asp:ListItem>
<asp:ListItem Text="" Value=""></asp:ListItem>
</asp:DropDownList>
</td>
<td>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ErrorMessage="*" ControlToValidate="ddlCourse" ForeColor="#990000"></asp:RequiredFieldValidator></td>
</tr>
<tr>
<td class="auto-style1"></td>
<td class="auto-style1">
<asp:Button ID="btnSearch" runat="server" Text="SEARCH" CssClass="button" OnClick="btnSearch_Click" Height="34px" Width="77px" /></td>
<td colspan="2" class="auto-style1">
<asp:Button ID="btnRefresh" runat="server" Text="REFRESH" CssClass="button" OnClick="btnRefresh_Click" Height="34px" Width="77px" /></td>
</tr>
</table>
</div>
</div>
<asp:GridView ID="GVdata" runat="server" Width="674px" CaptionAlign="Top"
AutoGenerateColumns="False" Height="100px" BackColor="White"
BorderColor="#999999" BorderStyle="None" BorderWidth="1px" CellPadding="3"
CellSpacing="2" GridLines="Vertical"
EmptyDataText="There Are No Record Found" OnRowCancelingEdit="gvManageOrders_RowCancelingEdit"
OnRowDeleting="gvManageOrders_RowDeleting" OnRowEditing="gvManageOrders_RowEditing"
OnRowUpdating="gvManageOrders_RowUpdating">
<AlternatingRowStyle BackColor="#DCDCDC" />
<Columns>
<asp:TemplateField HeaderText="Sr. No." ItemStyle-Width="20">
<ItemTemplate>
<asp:Label ID="lblRowNumber" Text='<%# Container.DataItemIndex + 1 %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:CommandField HeaderText="Edit/Remove" ShowDeleteButton="True"
ShowEditButton="True" DeleteText="Remove" />
<asp:TemplateField HeaderText="Student ID" Visible="True">
<ItemTemplate>
<asp:Label ID="studentID" runat="server" Text='<%# Bind("studentID")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Student Name" Visible="True">
<ItemTemplate>
<asp:Label ID="Sname" runat="server" Text='<%# Bind("Sname")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Father's Name" Visible="True">
<ItemTemplate>
<asp:Label ID="Fname" runat="server" Text='<%# Bind("Fname")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
<EmptyDataRowStyle BackColor="#eeeeee" BorderColor="Black"
BorderStyle="Solid" BorderWidth="1px" Font-Size="Large" ForeColor="#851010"
HorizontalAlign="Center" />
<FooterStyle BackColor="#CCCCCC" ForeColor="Black" />
<HeaderStyle BackColor="#851010" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#851010" ForeColor="Black" HorizontalAlign="Center" />
<RowStyle BackColor="#EEEEEE" ForeColor="Black" HorizontalAlign="Center" />
<SelectedRowStyle BackColor="#008A8C" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F1F1F1" />
<SortedAscendingHeaderStyle BackColor="#0000A9" />
<SortedDescendingCellStyle BackColor="#CAC9C9" />
<SortedDescendingHeaderStyle BackColor="#000065" />
</asp:GridView>
</fieldset>
</div>
</asp:Content>
and here is my vb code:
Imports System.Collections.Generic
Imports System.Configuration
Imports System.Data.SqlClient
Imports System.Globalization
Imports System.Linq
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.Net.Mail
Imports System.Data
Imports System.ComponentModel
Imports System.Drawing
Imports System.Text
Imports System.Windows.Forms
Partial Public Class Admin_EditSTUdetail
Inherits System.Web.UI.Page
Private myds As DataSet
Protected Sub LoadSession()
Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("cmsDB").ConnectionString)
con.Open()
Dim com As New SqlCommand("select sessionID, session from tblcategories ORDER by session DESC", con)
Dim da As New SqlDataAdapter(com)
Dim ds As New DataSet()
da.Fill(ds)
ddlSession.DataTextField = ds.Tables(0).Columns("session").ToString()
' text field name of table dispalyed in dropdown
ddlSession.DataValueField = ds.Tables(0).Columns("sessionID").ToString()
' to retrive specific textfield name
ddlSession.DataSource = ds.Tables(0)
'assigning datasource to the dropdownlist
ddlSession.DataBind()
ddlSession.SelectedIndex = -1
'binding dropdownlist
End Sub
Protected Sub LoadCourse()
Dim con As New SqlConnection(ConfigurationManager.ConnectionStrings("cmsDB").ConnectionString)
con.Open()
Dim com As New SqlCommand("SELECT DISTINCT course AS mycolumn FROM " _
& " tblsubjects where course is not null and " _
& "not course = '' order by mycolumn", con)
Dim da As New SqlDataAdapter(com)
Dim ds As New DataSet()
da.Fill(ds)
ddlCourse.DataTextField = ds.Tables(0).Columns("mycolumn").ToString()
' text field name of table dispalyed in dropdown
ddlCourse.DataValueField = ds.Tables(0).Columns("mycolumn").ToString()
' to retrive specific textfield name
ddlCourse.DataSource = ds.Tables(0)
'assigning datasource to the dropdownlist
ddlCourse.DataBind()
ddlCourse.SelectedIndex = -1
'binding dropdownlist
End Sub
Protected Sub fillgrid()
Dim cn As New SqlConnection(ConfigurationManager.ConnectionStrings("cmsDB").ConnectionString)
Dim cmd As New SqlCommand("SELECT StudentID,session,course,Sname,Fname FROM [tblstudetail] WHERE " _
& " [session] = '" + ddlSession.SelectedValue.ToString() + "' AND " _
& " [course] = '" + ddlCourse.SelectedValue.ToString() + "'", cn)
cn.Open()
Dim da As New SqlDataAdapter(cmd)
myds = New DataSet()
da.Fill(myds)
GVdata.DataSource = myds
GVdata.DataBind()
End Sub
Private Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
LoadSession()
LoadCourse()
fillgrid()
End If
End Sub
Protected Sub gvManageOrders_RowCancelingEdit(sender As Object, e As GridViewCancelEditEventArgs)
Throw New NotImplementedException
End Sub
Protected Sub gvManageOrders_RowDeleting(sender As Object, e As GridViewDeleteEventArgs)
Throw New NotImplementedException
End Sub
Protected Sub gvManageOrders_RowEditing(sender As Object, e As GridViewEditEventArgs)
Throw New NotImplementedException
End Sub
Protected Sub gvManageOrders_RowUpdating(sender As Object, e As GridViewUpdateEventArgs)
Throw New NotImplementedException
End Sub
Protected Sub btnSearch_Click(sender As Object, e As EventArgs) Handles btnSearch.Click
If Not Page.IsPostBack Then
fillgrid()
End If
End Sub
Protected Sub btnRefresh_Click(sender As Object, e As EventArgs) Handles btnRefresh.Click
GVdata.DataSource = Nothing
End Sub
End Class
But it is not returning what i request.
Kindly HELP me out this....

Answer 1

PLease remove these lines ,
ddlSession.SelectedIndex = -1
ddlCourse.SelectedIndex = -1
from the end of the LoadCourse and LoadSession methods. These are resetting the dropdown values and causing issue.
The flow is messed up in your code. You are calling loadsession , loadcourses and fillgrid serially from the page load method. Think about it. When the code in fillgrid executes there is no selected value in the drop downs. Obviously no data is found and so the gridview is not visible.
The better way to handle is introduce two new methods like this,
Protected Sub ddlCourses_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles ddlSession.SelectedIndexChanged
and
Protected Sub ddlAC3_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles ddlCourse.SelectedIndexChanged
Inside these methods send out a call to fillgrid.
Also inside the fillgrid method first validate there is some value in both the ddlCourse and ddlSession dropdowns , then send out the database command.

Answer 2

First of all
ddlSession.SelectedValue.ToString()
become
ddlSession.SelectedValue
Then in Protected Sub btnSearch_Click remove
If Not Page.IsPostBack Then
Then
ddlCourse.SelectedIndex = -1
become
ddlCourse.ClearSelection();
Finally, are you sure there aren't NULL value in the database?