how to Validating blank textbox in repeater?

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C#2.0

C#

hi everybody

In my project ,I am required to make a validating work:

I have a repeater (.net 2.0,C#,VS 2k5),In this repeater
there are a column of Textbox,
a column of Button that using to editing value of textbox,
and a column of Checkbox that using to assign for selecting row
Outsite Repeater a have a Button ,when I click this one,all of row selected by checkbox will update in database
I must do how to selected Rows will not update and raise a validate at Rows that contain blank textbox, focus to which blank textbox
Please help me .Thank very much

My ASP:

<asp:Repeater ID="rptOrder" runat="server" OnItemCommand="Action" OnItemDataBound="rptOrder_ItemDataBound">
            <itemtemplate>
                <tr class="DataRowStyle CursorHand smaller" title="Upload on : [<%#Eval("uploaddate","{0:MM/dd/yyyy}")%>] - Process On:[<%#Eval("RecordDate","{0:MM/dd/yyyy}")%>] - Upload by:[<%#Eval("uploader")%>]- Process by: [<%#Eval("processer")%>]- Undoprocess by: [<%#Eval("processerUndo")%>]-FOC Qty:[<%#Eval("TotalQty")%>]; ">
                    <td align="left">
                        <asp:ImageButton ID="ImageButton1" runat="server" CommandArgument='<%#Eval("OrderID")%>'
                            CommandName="detail" SkinID="View" OnCommand="OnViewOrderDetail" OnDataBinding="btnEdit_DataBinding" />
                        <asp:Label ID="lbOrderId" runat="server" Text='<%#Eval("OrderID")%>' Visible="false">
                    </td>
                    <td align="left" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("SupplierName")%>
                    </td>
                    <td align="left" önclick="<%#GetPostBackScript()%>">
                        <%#Eval("program")%>
                    </td>
                    <td align="left" önclick="<%#GetPostBackScript()%>">
                        <%#Eval("CustCode")%>
                    </td>
                    <td align="left" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("CustName") %>
                    </td>
                    <td align="left" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("Address") %>
                    </td>
                    <td align="right" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("District") %>
                    </td>
                    <td align="right" önclick="<%#GetPostBackScript()%>">
                        <%#Eval("Amount","{0:#,###}")%>
                    </td>
                    <td align="right">
                        <asp:TextBox ID="txtsno" Visible="false" runat="server" Text='<%#Eval("SONo")%>'
                            Width="98%">
                        <asp:Literal ID="ltsno" runat="server" Text='<%#Eval("SONo")%>'>
                        <asp:Literal ID="ltvalidate" runat="server" Text='invalid' Visible="false">
                    </td>
                    <td align="right" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("FOCValue","{0:#,###}")%>
                    </td>
                    <td align="right" önclick="<%#GetPostBackScript()%>">
                        <%# Eval("Status")%>
                    </td>
                    <td align="right">
                        <%--style="visibility:<%#Eval("sttchk")%>;--%>
                        <asp:CheckBox ID="chkCheck" runat="server" />
                    </td>
                    <td align="right">
                        <asp:Button ID="btnEdit" runat="server" Text="Edit" CommandArgument="argUpdate" CommandName="Action" />
                        <asp:Button ID="btnCancel" runat="server" Text="Cancel" CommandArgument="argCancel"
                            Visible="false" CommandName="Action" />
                    </td>
                </tr>
            </itemtemplate>

My button:

<asp:LinkButton ID="lnkProcess" runat="server" OnClick="lnkProcess_Click" Font-Bold="True" CommandName ="Action" CommandArgument ="p">PROCESS

MyCode Behind:

protected void lnkProcess_Click(object sender, EventArgs e)
        {
            for (int i = 0; i < rptOrder.Items.Count; i++)
            {
                 RepeaterItem items = rptOrder.Items[i];
                if (((CheckBox)items.FindControl("chkcheck")).Checked)
                {
                    string oid = ((Label)items.FindControl("lbOrderId")).Text;
                    if (((TextBox)items.FindControl("txtsno")).Text != "")
                    {
                        BlueOcean.Data.BonusPayment.BPClass.BPOrderProcess(oid);
                    }
                    else
                    {
//I must do how to validate ........................help me
                    }
                }
            }
            LoadData();
        }