Fire TextChanged event for GridView inside TextBox in ASP.Net

This way

    <asp:GridView ID="GridView1" runat="server"  AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:BoundField DataField="Country" HeaderText="Country" />
            <asp:TemplateField>
                <ItemTemplate>
                    <asp:TextBox runat="server" OnTextChanged = "OnTextChanged" AutoPostBack = "true" />  
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id", typeof(int)),
                            new DataColumn("Name", typeof(string)),
                            new DataColumn("Country",typeof(string)) });
            dt.Rows.Add(1, "John Hammond", "United States");
            dt.Rows.Add(2, "Mudassar Khan", "India");
            dt.Rows.Add(3, "Suzanne Mathews", "France");
            dt.Rows.Add(4, "Robert Schidner", "Russia");
            GridView1.DataSource = dt;
            GridView1.DataBind();
        }
    }
    protected void OnTextChanged(object sender, EventArgs e)
    {
        ClientScript.RegisterClientScriptBlock(GetType(), "alert", "alert('" + (sender as TextBox).Text + "');", true);
    }