WPF 组合框和文本框行为

问：

我做了一个简单的测试，试图找出我在组合框中遇到的问题。我想让用户有机会在触发计算之前取消对控件所做的编辑。

在测试中，一个文本框给了我想要的结果。取消时，控件将恢复到原来的状态。取消组合框中的输入时，组合框看起来好像没有取消。在这两种情况下，模型和视图模型都是正确的。

如何让组合框的行为方式与文本框的行为方式相同，以允许控件正确反映视图模型？

模型和视图模型：

Option Explicit On
Option Strict On
Imports System.ComponentModel

Public Class ViewModel
    Implements INotifyPropertyChanged
    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
    Protected Overridable Sub OnPropertyChanged(propertyName As String)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
    End Sub

    Public Sub New()
        Model = New Model
    End Sub

    Public Property Model As Model
    Public Property Text As String
        Get
            Return Model.Text
        End Get
        Set(value As String)
            If cancel() Then
                OnPropertyChanged("Text")
                Exit Property
            End If
            Model.Text = value
            OnPropertyChanged("Text")
        End Set
    End Property

    Private Function cancel() As Boolean
        If MsgBox("Cancel?", MsgBoxStyle.YesNo) = MsgBoxResult.Yes Then
            Return True
        Else Return False
        End If
    End Function
End Class

Public Class Model
    Public Sub New()
        Text = "Hello, World!"
    End Sub
    Public Property Text As String
End Class

视图：

<Window x:Class="MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:test"
        mc:Ignorable="d"
        Title="MainWindow" Height="241" Width="416" d:DataContext="{d:DesignInstance Type=local:ViewModel}">
    <Grid>
        <TextBox HorizontalAlignment="Left" Margin="108,95,0,0" TextWrapping="Wrap" Text="{Binding Text, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="120"/>
        <ComboBox HorizontalAlignment="Left" Margin="108,144,0,0" VerticalAlignment="Top" Width="120" IsEditable="True" Text="{Binding Text, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
    </Grid>
</Window>

窗口代码隐藏：

Class MainWindow
    Public Sub New()
        InitializeComponent()
        DataContext = New ViewModel
    End Sub
End Class