检查是否至少选中了一个复选框列表

I have checkboxlist and I would like to check if at least one checkbox is checked. If none is checked then I want to show alert message that says please select at least one item. I want to do this in code behind if possible. I have started but don't know if it is right or wrong but not able to finish it. I have checkboxlist and I would like to check i