WPF PropertyChanged为空

Question

我在代码隐藏文件中有一个属性。

private int? selectedTypeID = null;
public int? SelectedTypeID
{
  get
  {
    return selectedTypeID;
  }
  set
  {
    selectedTypeID = value;
    OnPropertyChanged( "SelectedTypeID" );
  }
}

这是PropertyChanged的代码。在注释行中提到了这个问题，请参阅。

#region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    public void OnPropertyChanged( string propertyName )
    {
      /*PropertyChanged appears to be null in some cases while in some cases it is not null. I have also tried to explicity assigning it the DataContext but that does not work as well. */
      if( PropertyChanged != null )
        PropertyChanged( this, new PropertyChangedEventArgs( propertyName ) );
    }

#endregion


//This line is in DataContext file where the problematic property is assigned a null.
editLayerSidebar.editConditionIngredient.SelectedTypeID = null;

//This is the combobox xaml where SelectedTypeID has been bound to SelectedValue.
<ComboBox x:Name="TypeCombo" Grid.Row="3" Grid.Column="1" Margin="5,5,0,0" 
                  ItemsSource="{Binding DataContext.IngredientTypes, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:EditConditionListLayer}}}"
                  DisplayMemberPath="Name" SelectedValuePath="ID" SelectedValue="{Binding SelectedTypeID, RelativeSource={RelativeSource Mode=Self}}" >

为什么ProperyChanged经常会变成空，导致不能更新组合框？解决方案应该是什么？

Answer 1

我相信您绑定的SelectedValue是错误的。您的ComboBox本身没有任何名为SelectedTypeID的属性。它应该是视图模型的一些属性。在这种情况下，RelativeSource应该沿着可视化树向上遍历，以具有您想要的DataContext的源为目标(在这种情况下，我猜它的类型是local:EditConditionListLayer)，然后Path应该以DataContext为前缀：

SelectedValue="{Binding DataContext.SelectedTypeID, 
     RelativeSource={RelativeSource AncestorType={x:Type local:EditConditionListLayer}}}"

此外，我怀疑你的ComboBox本身是否有你想要的DataContext，如果是的话，它可能是：

SelectedValue="{Binding DataContext.SelectedTypeID, 
                        RelativeSource={RelativeSource Self}}"