组合框空引用异常上的 Selecteditem

Question

我想使用 SelectedItem 将选择设置为代码中的组合框。 我只能通过使用 SelectedValue 让它工作。 SelectedItem 将在堆栈跟踪的顶部抛出空引用异常：

在 AttachedCommandBehavior.CommandBehaviorBinding.Execute()

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

公共类 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

结束类

为什么选择项不起作用？

更新：

尼古拉：你的眼光很敏锐。那是由于最后一分钟的复制粘贴工作！

Blindmeis：当然，这是对一个更大的程序的抽象，在该程序中我需要 selectionchanged 事件来执行一些操作。这些命令绑定必须保留（尽管可能需要一些修复）。

问候，

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 ;)

这行得通。

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}"

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