为什么我不能在ComboBox中选择空值?

In WPF, it seems to be impossible to select (with the mouse) a "null" value from a ComboBox. Edit To clarify, this is .NET 3.5 SP1.

在WPF中，似乎不可能(用鼠标)从ComboBox中选择“null”值。编辑澄清，这是。net 3.5 SP1。

Here's some code to show what I mean. First, the C# declarations:

这里有一些代码来说明我的意思。首先,c#声明:

public class Foo
{
    public Bar Bar { get; set; }
}

public class Bar 
{
    public string Name { get; set; }
}

Next, my Window1 XAML:

接下来,我的Window1 XAML:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <StackPanel>
        <ComboBox x:Name="bars" 
                  DisplayMemberPath="Name" 
                  Height="21" 
                  SelectedItem="{Binding Bar}"
                  />
    </StackPanel>
</Window>

And lastly, my Window1 class:

最后，我的windows1类:

public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();

        bars.ItemsSource = new ObservableCollection<Bar> 
        {
            null, 
            new Bar { Name = "Hello" }, 
            new Bar { Name = "World" } 
        };
        this.DataContext = new Foo();
    }
}

With me? I have a ComboBox whose items are bound to a list of Bar instances, one of which is null. I have bound the window to an instance of Foo, and the ComboBox is displaying the value of its Bar property.

和我在一起吗?我有一个ComboBox，它的项被绑定到一个Bar实例列表，其中一个实例为null。我将窗口绑定到Foo实例，ComboBox显示其Bar属性的值。

When I run this app, the ComboBox starts with an empty display because Foo.Bar is null by default. That's fine. If I use the mouse to drop the ComboBox down and select the "Hello" item, that works too. But then if I try to re-select the empty item at the top of the list, the ComboBox closes and returns to its previous value of "Hello"!

当我运行这个应用程序时，ComboBox以一个空的显示开始，因为Foo。Bar默认为空。这很好。如果我使用鼠标来下拉ComboBox并选择“Hello”项，也可以。但是，如果我尝试重新选择列表顶部的空项，ComboBox将关闭并返回到之前的值“Hello”!

Selecting the null value with the arrow keys works as expected, and setting it programatically works too. It's only selecting with a mouse that doesn't work.

使用箭头键选择null值和预期的一样，并通过编程方式设置它。它只是用一个不能用的鼠标进行选择。

I know an easy workaround is to have an instance of Bar that represents null and run it through an IValueConverter, but can someone explain why selecting null with the mouse doesn't work in WPF's ComboBox?

我知道一个简单的解决方案是有一个Bar实例，它表示null，并通过IValueConverter运行它，但是有人能解释为什么用鼠标选择null在WPF的ComboBox中不起作用吗?

10 个解决方案

#1

The null "item" is not being selected by the keyboard at all - rather the previous item is being unselected and no subsequent item is (able to be) selected. This is why, after "selecting" the null item with the keyboard, you are thereafter unable to re-select the previously selected item ("Hello") - except via the mouse!

空“项目”根本不是由键盘选择的——而是前面的项目被取消选择，没有后续的项目被选中(可以被选中)。这就是为什么在用键盘“选择”空项后，您以后无法重新选择先前选中的项(“Hello”)——除了通过鼠标!

In short, you can neither select nor deselect a null item in a ComboBox. When you think you are doing so, you are rather deselecting or selecting the previous or a new item.

简而言之，您不能在组合框中选择或取消选中空项。当你认为你在这样做的时候，你是在取消选择或者选择之前的或者一个新的项目。

This can perhaps best be seen by adding a background to the items in the ComboBox. You will notice the colored background in the ComboBox when you select "Hello", but when you deselect it via the keyboard, the background color disappears. We know this is not the null item, because the null item actually has the background color when we drop the list down via the mouse!

这可以通过在ComboBox中添加一个背景来实现。当您选择“Hello”时，您将注意到ComboBox中的彩色背景，但是当您通过键盘取消选中它时，背景颜色将消失。我们知道这不是空项目，因为空项目实际上有背景颜色，当我们通过鼠标向下拖拽列表!

The following XAML, modified from that in the original question, will put a LightBlue background behind the items so you can see this behavior.

下面的XAML，在最初的问题中修改过，将在项目后面添加一个LightBlue背景，这样您就可以看到这种行为。

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <StackPanel>
        <ComboBox x:Name="bars" Height="21" SelectedItem="{Binding Bar}">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <Grid Background="LightBlue" Width="200" Height="20">
                        <TextBlock Text="{Binding Name}" />
                    </Grid>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </StackPanel>
</Window>

If you want further validation, you can handle the SelectionChanged event on the ComboBox and see that "selecting the null item" actually gives an empty array of AddedItems in its SelectionChangedEventArgs, and "deselecting the null item by selecting 'Hello' with the mouse" gives an empty array of RemovedItems.

如果您希望进行进一步的验证，您可以处理ComboBox上的SelectionChanged事件，并看到“选择空项”实际上会在其SelectionChangedEventArgs中给出一个addeditem的空数组，而“用鼠标选择‘Hello’取消选中空项”则会得到一个removeditem的空数组。

#2

Well I recently ran into the same problem with null value for ComboBox. I've solved it by using two converters:

我最近遇到了ComboBox的空值问题。我用两个转换器解决了这个问题

For ItemsSource property: it replaces null values in the collection by any value passed inside converter's parameter:

对于ItemsSource属性:它使用转换器参数中传递的任何值替换集合中的空值:

class EnumerableNullReplaceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var collection = (IEnumerable)value;

        return
            collection
            .Cast<object>()
            .Select(x => x ?? parameter)
            .ToArray();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

For SelectedValue property: this one does the same but for the single value and in two ways:

对于SelectedValue属性:这个值与单个值相同，但有两种方式:

class NullReplaceConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value ?? parameter;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return value.Equals(parameter) ? null : value;
    }
}

Example of use: