因为您没有添加字符串，但是将ComboBoxItems添加到ComboBox，则还必须设置其 SelectedValuePath 属性：

Since you don't add strings, but ComboBoxItems to your ComboBox, you would also have to set its SelectedValuePath property:

<ComboBox SelectedValuePath="Content"
          SelectedValue="{Binding Source={x:Static properties:Settings.Default},
                                  Path=KeyModifier, Mode=TwoWay}">
    <ComboBoxItem>Alt</ComboBoxItem>
    <ComboBoxItem>Shift</ComboBoxItem>
    <ComboBoxItem>Ctrl</ComboBoxItem>
    <ComboBoxItem>Win</ComboBoxItem>
</ComboBox>

或者将字符串添加到ComboBox，并使用 SelectedItem 而不是 SelectedValue ：

Alternatively add strings to the ComboBox, and use SelectedItem instead of SelectedValue:

xmlns:sys="clr-namespace:System;assembly=mscorlib"
...
<ComboBox SelectedItem="{Binding Source={x:Static properties:Settings.Default},
                                 Path=KeyModifier, Mode=TwoWay}">
    <sys:String>Alt</sys:String>
    <sys:String>Shift</sys:String>
    <sys:String>Ctrl</sys:String>
    <sys:String>Win</sys:String>
</ComboBox>

还请注意，自WPF 4.5起，您可以编写这样的绑定：

Note also that since WPF 4.5 you may write the Binding like this:

SelectedItem="{Binding Path=(properties:Settings.Default).KeyModifier, Mode=TwoWay}"