GridView with SqlDataSource

<asp:Button ID="btn" runat="server" OnClick="btnView_Click" CausesValidation="true" Text="Search" />  

<asp:gridview id="gv"  allowpaging="true" allowsorting="true" runat="server" autogeneratecolumns="False" datakeynames="StudentID"  datasourceid="SqlDataSource1" >
<Columns>
        <asp:BoundField DataField="IDString" HeaderText="Student ID" SortExpression="IDString" />
</Columns>
</asp:gridview>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:************* %>"
            CancelSelectOnNullParameter="false" SelectCommand="Select=@*****"
            OnSelecting="on_Selecting">
            <SelectParameters>
                <asp:Parameter Name="*****" />
           </SelectParameters>
</asp:SqlDataSource>

   

 1  protected void on_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
 2     {
 3         string strFilter = GetFilterStr();
 4         SqlDataSource1.FilterExpression = strFilter;
 5     }
 6 
 7     private string GetFilterStr()
 8     {
 9         StringBuilder strFilter = new StringBuilder();
10         strFilter.Append("1=1");
11 
12         strFilter.AppendFormat(" and (XXXXX='{0}')", XXXX);
13     }
14 
15 
16 
17     protected void btnView_Click(object sender, EventArgs e)
18     {
19         SqlDataSource1.SelectParameters["SchoolID"].DefaultValue = SchoolDropDownList1.SelectedValue.ToString();
20         gv.DataBind();
21     }
22 
23