private void SetupColorComboBoxes() { // add colors starting with the letter A ColorsComboBox1.DataSource = ColorsBeginningWith("A"); // add colors starting with B, will not show up var bColors = ColorsBeginningWith("B"); ((List <nf.ColorItem>)ColorsComboBox1.DataSource).AddRange(bColors.ToArray()); /* * We need to use a BindingList to allow a ComboBox to recognize new * items added. */ var bColorList = new BindingList <nf.ColorItem>(ColorsBeginningWith("A")); ColorsComboBox2.DataSource = bColorList; bColorList.AddColors(bColors); }