GridView + Checkboxes

[mansi sharma]

I bound gridview with databse..property Checkbox -true .
I have three columns roll,name ,roll.

I want that on button_click, roll nos are displayed whose checkboxes are checked in gridview--
SOURCE TAB

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False">                     
   <Columns>                       
 <asp:TemplateField >  
    <ItemTemplate >                   
     <asp:CheckBox ID ="chk" runat ="server"  />                        </ItemTemplate>                
        </asp:TemplateField>             
   <asp:BoundField HeaderText="Roll" DataField ="roll" />                           <asp:BoundField HeaderText="Name"  DataField ="name"/>                       
 <asp:BoundField HeaderText="Marks" DataField ="marks"/>                        </Columns>                  
  </asp:GridView>

public partial class Default3 : System.Web.UI.Page
{
string query;
SqlConnection conn = new SqlConnection("Data Source=SQLEXPRESS;Initial catalog=mansi;Integrated Security=true");
SqlCommand cmd;
protected void Page_Load(object sender, EventArgs e)
{
query = "Select * from Information";
conn.Open();
cmd = new SqlCommand(query, conn);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds=new DataSet ();
da.Fill (ds );
GridView1.DataSource = ds;
GridView1.DataBind();
conn.Close();
}


protected void Button1_Click(object sender, EventArgs e)
{
bool bchecked;

for (int i = 0; i < this.GridView1.Rows.Count; i++)
{
bchecked = ((CheckBox)GridView1.Rows[i].FindControl("chk")).Checked;

}
}
}

Suppose i have three rows in Gridview & I Uncheck first & third row,
But everytime in varable bchecked false is coming...y so???Plz help me out..