将Nullable值绑定到WPF ComboBox的问题

Question

我将WPF ComboBox绑定到MyEnum类型的可空属性？(其中MyEnum是枚举类型)

我以编程方式填充ComboBox项目,如下所示:

// The enum type being bound to 
enum MyEnum { Yes, No }

// Helper class for representing combobox listitems
// (a combination of display string and value)
class ComboItem {
  public string Display {get;set}
  public MyEnum? Value {get;set}
}

private void LoadComboBoxItems()
{
  // Make a list of items to load into the combo
  var items = new List<ComboItem> {
    new ComboItem {Value = null, Display = "Maybe"},
    new ComboItem {Value = MyEnum.Yes, Display = "Yes"},
    new ComboItem {Value = MyEnum.No, Display = "No"},};

  // Bind the combo's items to this list.
  theCombo.ItemsSource = items;
  theCombo.DisplayMemberPath = "Display";
  theCombo.SelectedValuePath = "Value";
}

同样在代码隐藏中,我将DataContext设置为一个类的实例,该类具有一个名为TheNullableProperty的属性(对于此示例,无论如何),类型为MyEnum？.

Combo的SelectedValue的绑定在我的XAML文件中完成.

<ComboBox 
  Name="theCombo" 
  SelectedValue="{Binding Path=TheNullableProperty,
                          UpdateSourceTrigger=PropertyChanged}"/>

问题:

当bound属性的值最初为非null时,组合框会正确显示该值.

但是当bound属性的值最初为null时,组合框为空.

看起来数据绑定的每个方面都与首次显示组合框时的空值表示不同.

例如:您可以从下拉列表中选择Maybe,并将bound属性正确设置为null.只是初始加载失败了.也许我需要最初手动设置SelectedValue ...

我结束了做什么

• 通过转换器将隐藏的文本块数据绑定添加到底层可为空的枚举值,该转换器从可空的枚举转换为字符串(enum.ToString或"null").
• 使用'ComboItems'加载组合框,每个组合框都有一个字符串Label(显示在组合中)和一个字符串Value等于枚举值作为字符串(和null值为"null").

• 将组合框数据绑定到文本块.

  /// <summary>
  /// Convert from EnumeratedType? to string (null->"null", enum values->ToString)
  /// </summary>
  public class EnumConverter<T> : IValueConverter where T:struct 
  {
    public static string To(T? c)
    {
      if (c == null)
        return "null";
      return c.ToString();
    }
  
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
      return To((T?)value);
    }
  
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
      var s = (string) value;
      if (s == "null")
        return null;
      return (T?)Enum.Parse(typeof(T), s);
    }
  }
  
  public class MyEnumConverter : EnumConverter<MyEnum>
  {
  }
  
  public class ComboItem
  {
    public string Value { get; set; }
    public string Label { get; set; }
  
    public ComboItem(MyEnum? e, string label)
    {
      Value = MyEnumConverter.To(e);
      Label = label;
    }
  }
  
  static IEnumerable<ComboItem> GetItems()
  {
    yield return new ComboItem(null, "maybe");
    yield return new ComboItem(MyEnum.Yes, "yup");
    yield return new ComboItem(MyEnum.No, "nope");
  }
  
  private void SetupComboBox()
  {
    thecombo.ItemsSource = GetItems().ToList();
  }
  

Answer 1

您无法在WPF(至少3.5 SP1)中按设计绑定到空值.这意味着,当Source获取Null作为值时,您的Binding将自动被破坏,即使您为Source指定了有效值也无法工作.您需要通过Value Converter为您的绑定提供一些"心脏跳动"机制.因此,Converter将null值转换为Targets的某些"Undefined"(非null),并且能够将"Undefined"转换回null ...