how to search the records using textbox

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            bind();
        }
    }
    DataTable dt = new DataTable();
    void bind()
    {
        using (
              SqlConnection con =
                  new SqlConnection(ConfigurationManager.ConnectionStrings["testConnectionString"].ConnectionString))
        {

            SqlCommand cmd = new SqlCommand("select  *  from dbo.Orders ", con);
            SqlDataAdapter adpt = new SqlDataAdapter(cmd);
            adpt.Fill(dt);
            GridView1.DataSource = null;
            GridView1.DataSource = dt;
            GridView1.DataBind();
            ViewState.Add("dt", dt);
            cmd.Dispose();

        }
    }
    protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        GridView1.PageIndex = e.NewPageIndex;
        dt = (DataTable)ViewState["dt"];
        if (ViewState["flag"] != null)
        {
            if ((bool)ViewState["flag"] == true)
            {
                GridView1.DataSource = null;
                dt = (DataTable)ViewState["dt"];
                DataView dvwData = new DataView(dt);
                dvwData.RowFilter = "orderID =" + Convert.ToInt32(TextBox1.Text) + "";
                GridView1.DataSource = dvwData;
                GridView1.DataBind();
            }
        }
        else
        {
            bind();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        ViewState.Add("flag", true);
        dt = (DataTable)ViewState["dt"];
        DataView dvwData = new DataView(dt);
        dvwData.RowFilter = "orderID ="+Convert.ToInt32(TextBox1.Text)+"";
        GridView1.DataSource = dvwData;
        GridView1.DataBind();
    }

<div>
        <asp:GridView ID="GridView1" runat="server" AllowPaging="True"
            onpageindexchanging="GridView1_PageIndexChanging" PageSize="5">
        </asp:GridView>
        <br />
        <br />
        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
&nbsp;
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    </div>