允许在DropDown中过滤，同时在ComboBox中强制值

Question

我正在尝试获得一个组合框下拉列表，允许用户键入组合框以过滤下拉列表，但强制用户必须选择其中一个可用选项，而不能只输入他们想要的内容。我有下面的代码和

comboBoxProperties.DropDownStyle = DropDownStyle.DropDownList;

行应该只允许用户选择一个可用的选项，但我仍然能够输入我想要的任何内容并点击选项卡，该字段只保留我输入的内容。

USStateCollection states = new USStateCollection();
    states.LoadAll();
    states.Filter = states.AsQueryable().OrderBy(d => d.StateName);
    settings.Columns.Add(c =>
    {
        c.Caption = "State";
        c.FieldName = "State";
        c.Width = Unit.Percentage(10);
        c.Settings.FilterMode = ColumnFilterMode.Value;
        c.ColumnType = MVCxGridViewColumnType.ComboBox;
        var comboBoxProperties = c.PropertiesEdit as ComboBoxProperties;
        comboBoxProperties.DataSource = states;
        comboBoxProperties.TextField = "StateName";
        comboBoxProperties.ValueField = "StateAbbrev";
        comboBoxProperties.ValueType = typeof(System.String);
        comboBoxProperties.DropDownStyle = DropDownStyle.DropDownList;
        comboBoxProperties.IncrementalFilteringMode = IncrementalFilteringMode.Contains;
    });