在WPF中，实现ComboBox多选，并且可以根据输入值动态显示下拉框

在WPF中，实现ComboBox多选可以通过设置ComboBox的IsEditable属性为True，并使用一个TextBlock来显示选中项，同时在ComboBox的Popup部分添加一个ItemsControl用于显示下拉选项。当用户输入时，我们可以动态地过滤下拉选项。以下是一个简单的实现示例：

<ComboBox IsEditable="True"
          Text="{Binding SelectedText}"
          PreviewTextInput="ComboBox_PreviewTextInput"
          PreviewKeyDown="ComboBox_PreviewKeyDown">
    <ComboBox.Template>
        <ControlTemplate TargetType="{x:Type ComboBox}">
            <Grid>
                <ToggleButton x:Name="ToggleButton"
                              Content="{TemplateBinding SelectionBoxItem}"
                              IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                              ClickMode="Press"/>
                <ContentPresenter x:Name="ContentSite"
                                  IsHitTestVisible="False"
                                  Content="{TemplateBinding SelectionBoxItem}"
                                  ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                  Margin="3,0,0,0"
                                  VerticalAlignment="Center"
                                  HorizontalAlignment="Left"/>
                <Popup x:Name="Popup"
                       IsOpen="{TemplateBinding IsDropDownOpen}"
                       Placement="Bottom"
                       StaysOpen="False">
                    <Grid>
                        <Border x:Name="DropDownBorder"
                                Background="White"
                                BorderThickness="1"
                                BorderBrush="LightGray">
                            <ScrollViewer>
                                <ItemsControl x:Name="ItemsControl"
                                              ItemsSource="{TemplateBinding ItemsSource}"
                                              ItemTemplate="{TemplateBinding ItemTemplate}"
                                              SelectionMode="Multiple"/>
                            </ScrollViewer>
                        </Border>
                    </Grid>
                </Popup>
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger Property="HasItems" Value="False">
                    <Setter Property="Visibility" Value="Collapsed" TargetName="DropDownBorder"/>
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter Property="Foreground" Value="Gray"/>
                </Trigger>
                <DataTrigger Binding="{Binding Path=Text.Length, RelativeSource={RelativeSource Self}}" Value="0">
                    <Setter Property="SelectedIndex" Value="-1" />
                </DataTrigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </ComboBox.Template>
</ComboBox>

在ViewModel中需要为ComboBox绑定一个ItemsSource和一个SelectedText属性，以及处理PreviewTextInput和PreviewKeyDown事件。在事件处理程序中，我们可以动态地过滤下拉选项并更新选中项。

private ObservableCollection<string> _options = new ObservableCollection<string>();
public ObservableCollection<string> Options
{
    get { return _options; }
    set { _options = value; OnPropertyChanged(nameof(Options)); }
}

private string _selectedText;
public string SelectedText
{
    get { return _selectedText; }
    set { _selectedText = value; OnPropertyChanged(nameof(SelectedText)); }
}

private void ComboBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    var comboBox = sender as ComboBox;
    var text = comboBox.Text + e.Text;
    comboBox.ItemsSource = Options.Where(o => o.ToLower().Contains(text.ToLower()));
}

private void ComboBox_PreviewKeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.Enter)
    {
        var comboBox = sender as ComboBox;
        var selectedItems = comboBox.GetSelectedItems<string>();
        if (selectedItems.Any())
        {
            SelectedText = string.Join(",", selectedItems);
        }
        else
        {
            SelectedText = comboBox.Text;
        }
        comboBox.IsDropDownOpen = false;
    }
}

注意，在事件处理程序中，我们使用了一个扩展方法GetSelectedItems，它可以获取选中项的值列表。

public static class ComboBoxExtensions
{
    public static IEnumerable<T> GetSelectedItems<T>(this ComboBox comboBox)
    {
        var items = comboBox.ItemsSource as IEnumerable<T>;
        if (items == null) yield break;

        foreach (var item in items)
        {
            var container = comboBox.ItemContainerGenerator.ContainerFromItem(item) as ComboBoxItem;
            if (container != null && container.IsSelected)
            {
                yield return item;
            }
        }
    }
}