SelectedIndexプロパティは現在選択されているインデックスを取得または設定します。The SelectedIndex is based on a zero-based index, meaning that 0 represents the first C1ComboBoxItem, 1 represents the second C1ComboBoxItem, and so on.
In the image below, the SelectedIndex is set to 2, which selects the third C1ComboBoxItem.