c# - 疯狂使用 ComboBox SelectedItem

Question

我有一个数据绑定(bind)的ComboBox，如下所示:

<ComboBox Canvas.Left="5" Canvas.Top="5" IsEnabled="{Binding Path=ComboBoxEnabled}" ItemsSource="{Binding Path=Items}" SelectedItem="{Binding Mode=TwoWay, Path=SelectedItem}" Width="250">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock MaxWidth="{Binding Path=ActualWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}}" Text="{Binding}" TextTrimming="CharacterEllipsis"/>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>
<Label Canvas.Left="5" Canvas.Top="4" TextOptions.TextFormattingMode="Display" Content="No process instances have been found." Height="{DynamicResource {x:Static SystemParameters.WindowCaptionHeightKey}}" IsEnabled="False" Visibility="{Binding Path=WatermarkVisibility}" Width="250"/>
<Button Canvas.Right="5" Canvas.Top="5" Click="ClickRefresh" Content="Refresh" Width="75"/>

然后，在我的 MainWindow.xaml.cs 中:

public MainWindow()
{
    InitializeComponent();
    DataContext = m_ViewModel = new ViewModel();
}

private void ClickRefresh(Object sender, RoutedEventArgs e)
{
    m_ViewModel.Populate();
}

这是我的 ViewModel.cs:

public ProcessInstance SelectedItem
{
    get { return m_SelectedItem; }
    set
    {
        if (m_SelectedItem != value)
        {
            m_SelectedItem = value;
            NotifyPropertyChanged("SelectedItem");
        }
    }
}

public ObservableCollection<ProcessInstance> Items
{
    get { return m_Items; }
    private set
    {
        if (m_Items != value)
        {
            m_Items = value;
            NotifyPropertyChanged("Items");
        }
    }
}

public ViewModel()
{
    Populate();
}

public void Populate()
{
    ProcessInstance selectedItem = m_SelectedItem;

    SelectedItem = null;
    Items = null;

    List<ProcessInstance> processInstances = new List<ProcessInstance>();

    foreach (Process process in Process.GetProcesses())
    {
        if (...)
            processInstances.Add(new ProcessInstance(process));
    }

    if (processInstances.Count == 0)
    {
        ComboBoxEnabled = false;
        WatermarkVisibility = Visibility.Visible;
    }
    else
    {
        Items = new ObservableCollection<ProcessInstance>(processInstances.OrderBy(x => x.Process.Id));

        if (selectedItem != null)
            SelectedItem = m_Items.SingleOrDefault(x => x.ProcessEquals(selectedItem));

        if (m_SelectedItem == null)
            SelectedItem = m_Items[0];

        ComboBoxEnabled = true;
        WatermarkVisibility = Visibility.Hidden;
    }
}

这是我的 ProcessInstance 类相关代码:

public override Boolean Equals(Object obj)
{
    return Equals(obj as ProcessInstance);
}

public override Int32 GetHashCode()
{
    Int32 hashCode;

    if ((m_Process == null) || m_Process.HasExited)
        hashCode = 0;
    else
    {
        hashCode = (m_Process.Id.GetHashCode() * 397) ^ m_Process.MainModule.BaseAddress.GetHashCode();

        if (!String.IsNullOrEmpty(m_Process.MainWindowTitle))
            hashCode = (hashCode * 397) ^ m_Process.MainWindowTitle.GetHashCode();
    }

    return hashCode;
}

public override String ToString()
{
    String processId = process.Id.ToString("X8", CultureInfo.CurrentCulture);
    String windowTitle = (process.MainWindowTitle.Length > 0) ? process.MainWindowTitle : "NULL";

    return String.Format(CultureInfo.CurrentCulture, "[{0}] {1} - {2}", type, processId, windowTitle);
}

public Boolean Equals(ProcessInstance other)
{
    if (other == null)
        return false;

    if (ReferenceEquals(this, other))
        return true;

    if (m_Process == null)
    {
        if (other.Process == null)
            return true;

        return false;
    }

    if (other.Process == null)
        return false;

    return ((m_Process.Id == other.Process.Id) && (m_Process.MainModule.BaseAddress == other.Process.MainModule.BaseAddress) && (m_Process.MainWindowTitle == other.Process.MainWindowTitle));
}

public Boolean ProcessEquals(ProcessInstance other)
{
    if (other == null)
        throw new ArgumentNullException("other");

    if (m_Process == null)
        return (other.Process == null);

    if (other.Process == null)
        return false;

    return ((m_Process.Id == other.Process.Id) && (m_Process.MainModule.BaseAddress == other.Process.MainModule.BaseAddress));
}

现在发生了什么......我在没有流程实例存在的情况下启动了应用程序:

然后我打开一个或多个流程实例并单击“刷新”按钮。 ComboBox 默认选择第一个...我可以保留那个选择或选择另一个，没关系:

现在我关闭每个流程实例并再次单击“刷新”按钮。在这种情况下，Populate() 方法将 SelectedItem 和 Items 都设置为 null，因此 ComboBox 看起来是空的，然后它禁用 ComboBox 并使水印 Label 可见。但这是我得到的:

之前的SelectedItem还在。为什么？为什么？!？!

[编辑] 这是一个下载项目的链接:

http://www.filedropper.com/damncb

Answer 1

您需要清除 ComboBox 上的 SelectedValue 属性，或者将 SelectedIndex 设置为 -1