How can i loop over all comboBoxes controls i have and add the items of each comboBox to a List?

I tried:

var comboBoxes = this.Controls
                  .OfType<ComboBox>()
                  .Where(x => x.Name.StartsWith("comboBox"));

            foreach (var cmbBox in comboBoxes)
            {
                CMBXWMICLASSES.AddRange(cmbBox.Items[
            }

But inside the loop the foreach how do i add the Items of each comboBox ?