为什么我在使用 MultiValueConverter 编辑后得到 UnsetValue？

Question

我有 DataGrid 和 DataGridComboBoxColumn 可以编辑的列。当我编辑它时抛出异常。调试后我发现Converter 收到DependencyProperty.UnsetValue，因此我必须通过返回"" 将其过滤掉，这导致显示空单元格。

我试图用谷歌搜索并理解为什么我得到DependencyProperty.UnsetValue 但没有运气。

有什么想法吗？

public UserControl()
{
    InitializeComponent();

    DataContext = new MyViewModel();
}

public class MyViewModel : ViewModelBase
{
    public ObservableCollection<MyItem> MyItems { get; private set; }

    public MyViewModel()
    {           
        LoadStates();
    }

    public void LoadStates()
    {
        MyItems = new ObservableCollection<MyItem>(DataProvider.GetList());

        //...
    }
}

public class MyItem
{
    public bool Start {get; set;}
}


public class BoolToStatusConverter : IMultiValueConverter
{
    private MyViewModel _model;            

    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        _model = values[0] as MyViewModel ;
        //TODO: Why I get UnsetValue??
        if (values[1] == DependencyProperty.UnsetValue) return "";

        //Using _model
        //...

        return (bool)values[1] ? Statuses.Start : Statuses.Stop;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        return new object[] { _model, (Statuses)value == Statuses.Start };
    }
}

<UserControl.Resources>
    <local:BoolToStatusConverter x:Key="BoolToStatusConverter" />

    <ObjectDataProvider x:Key="myEnum" MethodName="GetValues" ObjectType="{x:Type core:Enum}">
        <ObjectDataProvider.MethodParameters>
            <x:Type Type="local:Statuses"/>
        </ObjectDataProvider.MethodParameters>
    </ObjectDataProvider>

</UserControl.Resources>

<Grid>
    <DataGrid ItemsSource="{Binding MyItems}" AutoGenerateColumns="False" >
        <DataGrid.Columns>              
            <DataGridComboBoxColumn Header="Status" ItemsSource="{Binding Source={StaticResource myEnum}}">                    
                <DataGridComboBoxColumn.SelectedItemBinding>
                    <MultiBinding Converter="{StaticResource BoolToStatusConverter}">
                        <Binding Path="DataContext"
                                RelativeSource="{RelativeSource AncestorType={x:Type UserControl}}" />
                        <Binding Path="Start" />
                    </MultiBinding>
                </DataGridComboBoxColumn.SelectedItemBinding>
            </DataGridComboBoxColumn>
        </DataGrid.Columns>
    </DataGrid>
</Grid>

Answer 1

为什么不直接使用常规的 IValueConverter？我不确定在这种情况下您是否真的需要 IMultiValueConverter。这对我有用：

public class BoolToStatusConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if ((bool) value)
        {
            return Statuses.Start;
        }

        return Statuses.Stop;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var theValue = (Statuses)value;

        if (theValue == Statuses.Start)
        {
            return true;
        }

        return false;
    }
}

还有 XAML：

    <DataGrid ItemsSource="{Binding MyItems}" AutoGenerateColumns="False" >
        <DataGrid.Columns>
            <DataGridComboBoxColumn Header="Status" ItemsSource="{Binding Source={StaticResource myEnum}}">
                <DataGridComboBoxColumn.SelectedItemBinding>
                    <Binding Converter="{StaticResource BoolToStatusConverter}" Path="Start">
                    </Binding>
                </DataGridComboBoxColumn.SelectedItemBinding>
            </DataGridComboBoxColumn>
        </DataGrid.Columns>
    </DataGrid>

另外，为了进一步简化，只需将 MyItem 中的 Start 字段设置为 Statuses，然后完全松开转换器。