Selecteditem上的combobox null引用异常

Question

我想使用SelectedItem从代码中选择组合框。 我只能使用SelectedValue让它工作。 SelectedItem将在stacktrace的顶部抛出一个null引用异常：

AttachedCommandBehavior.CommandBehaviorBinding.Execute（）

中的

XAML：

<Window x:Class="MainWindowView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:acb="clr-namespace:AttachedCommandBehavior;assembly=AttachedCommandBehavior"
Title="Window1" Height="300" Width="300">
<StackPanel>
    <ComboBox Name="ComboItems1"
              DisplayMemberPath="Value" 
              SelectedValuePath="Key"
              ItemsSource="{Binding Items}" 
              SelectedValue="{Binding SelectedValue}" 
              acb:CommandBehavior.Event="SelectionChanged" 
              acb:CommandBehavior.Command="{Binding Path=SelectionChangedCommand}" 
              acb:CommandBehavior.CommandParameter="{Binding ElementName=ComboItems1, Path=SelectedItem}" />

    <ComboBox Name="ComboItems2"
              DisplayMemberPath="Value" 
              SelectedValuePath="Key"
              ItemsSource="{Binding Items}" 
              SelectedItem="{Binding SelectedItem}" 
              acb:CommandBehavior.Event="SelectionChanged" 
              acb:CommandBehavior.Command="{Binding Path=SelectionChangedCommand}" 
              acb:CommandBehavior.CommandParameter="{Binding ElementName=ComboItems2, Path=SelectedItem}"/>
</StackPanel>

代码：

Imports AttachedCommandBehavior

Public Class MainWindowViewModel

Private _mainWindowView As MainWindowView

Public Property Items As New List(Of KeyValuePair(Of Integer, String))
Public Property SelectedItem As Nullable(Of KeyValuePair(Of Integer, String))
Public Property SelectedValue As Nullable(Of Integer)
Public Property SelectionChangedCommand As ICommand

Public Sub New()

    Items.Add(New KeyValuePair(Of Integer, String)(1, "first item"))
    Items.Add(New KeyValuePair(Of Integer, String)(2, "second item"))
    Items.Add(New KeyValuePair(Of Integer, String)(3, "third item"))

    Dim simpleCommand As SimpleCommand = New SimpleCommand()
    simpleCommand.ExecuteDelegate = Sub(selectedItem As Object)
                                        HandleSelectionChanged(selectedItem)
                                    End Sub
    SelectionChangedCommand = simpleCommand

    SelectedValue = 1
    'SelectedItem = Items(1) 'uncomment this to raise the null ref exception

End Sub

Private Sub HandleSelectionChanged(ByRef selectedItem As Object)
    If selectedItem IsNot Nothing Then
        'Do something
    End If
End Sub

结束班

为什么选择项不起作用？

更新

尼古拉：你有敏锐的眼光。这是由于最后一刻复制粘贴工作！

Blindmeis：当然，这是一个来自更大程序的摘要，我需要selectchanged事件来执行某些操作。那些命令绑定必须保留（尽管可能需要一些修复）。

此致

米歇尔

Answer 1

为什么你有这些命令绑定？

    <ComboBox 
          DisplayMemberPath="Value" 
          SelectedValuePath="Key"
          ItemsSource="{Binding Items}" 
          SelectedItem="{Binding SelectedItem}" />

视图模型

    //this select the "third item" in your combobox
    SelectedItem = Items[2];/dont know the vb indexer stuff ;)

这有效。

编辑：

视图模型

     public KeyValuePair<int, string> SelectedItem
     {
        get{return this._selectedItem;}
        set{

           if(this._selectedItem==value)
               return;//no selection change

           //if you got here then there was a selection change
           this._selectedItem=value;
           this.OnPropertyChanged("SelectedItem");
           //do all action you want here
           //and you do not need selection changed event commmandbinding stuff

         }
     }      

Answer 2

acb:CommandBehavior.CommandParameter="{Binding ElementName=ComboItems, Path=SelectedItem}"

您没有名称为ComboItems的元素，您有ComboItems1和ComboItems2。我认为这是问题所在。