如何批量读取网页gridview里面的textbox值

这是我的后台C#代码：
  
for (int i = 0; i < count_blank; i++)
            {
                TextBox tb = (TextBox)Repeater2.Items[i].FindControl("txt_blank");
            }
但是为什么取不到textbox里面的值呢？这是前台gridview的代码   <asp:Repeater ID="Repeater2" runat="server">
                <ItemTemplate>
                                <table class="style2">
                                    <tr>
                                        <td>
                                            <table class="style2">
                                                <tr>
                                                    <td width="10px">
                                                    <asp:Label ID="Label4" runat="server" Text='<%# Eval("Index") %>'></asp:Label>
                                                    </td>
                                                    <td>
                                                        <asp:Label ID="Label6" runat="server" Text='<%# Eval("Base") %>'></asp:Label>
                                                    </td>
                                                </tr>
                                            </table>
                                        </td>
                                    </tr>
                                    <tr>
                                        <td>
                                            学生答案：<asp:Label ID="Label7" runat="server" Text='<%# Eval("StudentAnswer") %>'></asp:Label>
                                        </td>
                                    </tr>
                                    <tr>
                                        <td>
                                            正确答案：<asp:Label ID="Label8" runat="server" Text='<%# Eval("BlankAnswer") %>'></asp:Label>
                                        </td>
                                    </tr>
                                                                        <tr>
                                        <td>
                                            本题总分：<asp:Label ID="Labe9" runat="server" Text='<%# Eval("Score") %>'></asp:Label>
                                        </td>
                                    </tr>
                                    <tr>
                                        <td>
                                            教师评分：<asp:TextBox ID="txt_blank" runat="server"  
                                                ></asp:TextBox>
                                        </td>
                                    </tr>
                                </table>
                                <br />
                            </ItemTemplate>
                </asp:Repeater>