Tool绑定： 绑定到ComboBox的SelectValue

<ComboBox Name="cmb_WHSCD" VerticalAlignment="Center" Width="100" IsEditable="True"  >
    <ComboBox.ToolTip>
        <ToolTip DataContext="{Binding Path=PlacementTarget,RelativeSource={x:Static RelativeSource.Self}}">
            <TextBlock Text="{Binding  Path=SelectedValue,StringFormat={}退仓到：{0}}"/>
        </ToolTip>
    </ComboBox.ToolTip>
</ComboBox>