WPF组合框没有绑定更改

I have 3 combo boxes

我有3个组合框

<Grid>
   <ComboBox Name="cbo1" SelectionChanged="OnComboBoxChanged" />
   <ComboBox Name="cbo2" SelectionChanged="OnComboBoxChanged"/>
   <ComboBox Name="cbo3" SelectionChanged="OnComboBoxChanged" />
<Grid>
   <Co