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

```html
<%@ 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">&nbsp;&nbsp;
                           <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....