foreach(Control ctl in this.Controls)
            {
                CheckBox ck = ctl as CheckBox;
                if (ck != null)
                    ck.Checked = true;
            }

------解决方案--------------------
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C# code

            foreach(Control ctl in this.Controls)
            {
                CheckBox ck = ctl as CheckBox;
                if (ck != null)
                    ck.Checked = true;
  ……