Checkbox state maintains but fall when paging performed

Question

code behind:

protected void CheckBox1_CheckedChanged(object sender, EventArgs e)

{

CheckBox chk = (CheckBox)sender;

if (chk.Checked)

{

Session[chk.UniqueID] = true;

}

else

{

Session.Remove(chk.UniqueID);

}

}

protected void CheckBox1_DataBinding(object sender, EventArgs e)

{

CheckBox chk = (CheckBox)sender;

chk.Checked = Session[chk.UniqueID] != null;

}

aspx:file:

<asp:TemplateField HeaderText="Select" >

<ItemTemplate>

<asp:CheckBox ID="chkSelect" runat="server" OnCheckedChanged="CheckBox1_CheckedChanged"

OnDataBinding="CheckBox1_DataBinding" ></asp:CheckBox>

</ItemTemplate>

</asp:TemplateField>

 Checkbox state maintains but fall when paging performed by means selects same rows in page it automatically select another page 1 row again how to slove this by coing