Question

How to create combobox control with non-selectable items? For example, such groupnames or categorynames which visually divide items in dropdownlist into some groups or categories.

Answer 1

Instead of adding strings to your combobox you could add a special class and use selected item to determine whether the item is selected or not.

public partial class Form1 : Form
{
    private class ComboBoxItem
    {
        public int Value { get; set; }
        public string Text { get; set; }
        public bool Selectable { get; set; }
    }

    public Form1() {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e) {
        this.comboBox1.ValueMember = "Value";
        this.comboBox1.DisplayMember = "Text";
        this.comboBox1.Items.AddRange(new[] {
            new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=0},
            new ComboBoxItem() { Selectable = true, Text="Selectable1", Value=1},
            new ComboBoxItem() { Selectable = true, Text="Selectable2", Value=2},
            new ComboBoxItem() { Selectable = false, Text="Unselectable", Value=3},
            new ComboBoxItem() { Selectable = true, Text="Selectable3", Value=4},
            new ComboBoxItem() { Selectable = true, Text="Selectable4", Value=5},
        });

        this.comboBox1.SelectedIndexChanged += (cbSender, cbe) => {
            var cb = cbSender as ComboBox;

            if (cb.SelectedItem != null && cb.SelectedItem is ComboBoxItem && ((ComboBoxItem) cb.SelectedItem).Selectable == false) {
                // deselect item
                cb.SelectedIndex = -1;
            }
        };
    }
}