How to filter Gridview data using Linq Query

Question

 Select :<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true">

                            <asp:ListItem Text="---Select---"></asp:ListItem>

                            <asp:ListItem Text="StudentName"></asp:ListItem>

                            <asp:ListItem Text="Email"></asp:ListItem>

                            <asp:ListItem Text="Mobile"></asp:ListItem>

                </asp:DropDownList>

        <asp:TextBox ID="txtSearch" runat="server"></asp:TextBox>

        <asp:Button ID="btnSearch" runat="server" Text="Search" OnClick="btnSearch_Click" />

    <div>

        <asp:GridView ID="GridView1" runat="server"></asp:GridView>

    </div>

private void BindGrid()

{

var query = from p in db.StudentTables select new { p.StudentID, p.StudentName,p.Mobile,p.Email };          

            GridView1.DataSource = query;

            GridView1.DataBind();

}