下拉框不用对象作为绑定对象

<dxe:ComboBoxEdit  x:Name="comb_type"
                               Height="25"
                               ShowSizeGrip="False"
                               IsTextEditable="False"
                               Width="100"
                               Text="{Binding ImportType, Mode=TwoWay}"
                               VerticalAlignment="Center">
                <dxe:ComboBoxEditItem Content="余额更新" />
                <dxe:ComboBoxEditItem Content="其他信息(F3)" />
                <dxe:ComboBoxEditItem Content="更新月供" />
                <dxe:ComboBoxEditItem Content="申请全款" />
                <dxe:ComboBoxEditItem Content="返佣" />
                <dxe:ComboBoxEditItem Content="前排还款" />
            </dxe:ComboBoxEdit>

        //导入类型
        private string _importType;
        /// <summary>
        /// 导入类型
        /// </summary>
        public string ImportType
        {
            get
            {
                if (string.IsNullOrEmpty(_importType))
                {
                    _importType = "";
                }
                return _importType;
            }
            set
            {
                _importType = value;
                NotifyPropertyChanged(x => x.ImportType);
            }
        }