默认是这样的

点击后是这样的

如何让他默认就显示下拉框呢
<DataGridComboBoxColumn Header="数量" TextBinding="{Binding CurrentStyle}" MinWidth="100" >
<DataGridComboBoxColumn.ElementStyle >
<Style TargetType="ComboBox">
<Setter Property="ItemsSource" Value="{Binding MarkupStyle}" />
<Setter Property="SelectedValue" Value="{Binding ApplicableParty}" />
<Setter Property="SelectedValuePath" Value="{Binding CurrentParty}" />
<Setter Property="DisplayMemberPath" Value="{Binding Style}" />
</Style>
</DataGridComboBoxColumn.ElementStyle>
<DataGridComboBoxColumn.EditingElementStyle >
<Style TargetType="ComboBox" >
<Setter Property="ItemsSource" Value="{Binding MarkupStyle}" />
<Setter Property="SelectedValue" Value="{Binding ApplicableParty}" />
<Setter Property="SelectedValuePath" Value="{Binding CurrentParty}" />
<Setter Property="DisplayMemberPath" Value="{Binding Style}" />
</Style>
</DataGridComboBoxColumn.EditingElementStyle >