How can I know the BId value of selected items in GridView control?

I have added a CheckBox control to a ItemTemplate of GridView control, so I can select the multi-row by checked CheckBox control in GridView.
How can I know the BId value of selected items in GridView control?





//----------------------------Program Code (You may save it as a ASPX file and run it)--------------------------------

<%@ Page Language="C#" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

<script runat="server">

</script>

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="XmlDataSource1">
<Columns>
<asp:TemplateField HeaderText="Check Box">
<ItemTemplate>
<asp:CheckBox ID="CheckBox1" runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="BId" HeaderText="BId" SortExpression="BId" />
<asp:BoundField DataField="Title" HeaderText="Title" SortExpression="Title" />
<asp:BoundField DataField="Url" HeaderText="Url" SortExpression="Url" />
<asp:BoundField DataField="InputDate" HeaderText="InputDate" SortExpression="InputDate" />
</Columns>
</asp:GridView>

</div>
<asp:XmlDataSource ID="XmlDataSource1" runat="server">
<Data>
<UserBookmarks><Res:Bookmark xmlns:Res="http://www.hellocw.com/onlinebookmark" BId="ghdce3ak-456c-4e38-ab2f-5h02d9711b67" Title="Welcome to ePassporte" Url="https://www.epassporte.com/" InputDate="2004-08-12" /><Res:Bookmark xmlns:Res="http://www.hellocw.com/onlinebookmark" BId="fkdfh3a8-456c-6y38-jk2f-5h0gh9711b45" Title="Keystone DreamCard" Url="https://www.mydreamcardonline.com" InputDate="2004-09-25" /></UserBookmarks></Data>
</asp:XmlDataSource>
</form>
</body>
</html>

//----------------------------Program Code (You may save it as a ASPX file and run it)--------------------------------