Getting selected CheckBoxList item

I have a checkboxlist. When I check item(s) and hit a button, all those items' Selected properties are still set to false. Here's my code: 
private void btnShow_Click(object sender, System.EventArgs e)
{
string strchklist="";
foreach (ListItem li in CheckBoxList1.Items )
{
if (li.Selected )
{
strchklist += li.Text + " " ;
}
}
}
I don't think there's a problem in this Click method...  Can anyone shine some light on this weird problem?

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