How to get id of checkBox whene button click?

You can iterate through on server side on your controls and check if the actual checkbox checked or not.

//your button click function
List<string> yourIds = new List<string>();
CheckBox cb;

foreach (object actualObject in Controls)
{
    if (actualObject.GetType() == typeof(CheckBox))
    {
       cb = actualObject as CheckBox;

       if (cb.Checked)
           yourIds.Add(cb.ID);
    }
}

You can do this on client side as well.