Question

当用户点击最后一行添加新行时，标准WPF DataGrid中存在一个已知错误。

抛出异常是因为在处理代表'NewItemPlaceholder'的MS.Internal.NamedObject时，ConvertBack方法（在默认转换器上）失败。如果CanUserAddRows设置为True（并且集合支持它），则此实例用于表示空白的“新行”。实际上，在尝试跟踪绑定失败时，似乎实际上在异常处理程序中抛出了FormatException。有关详细信息，请参阅Nigel Spencer's Blog。

基本上解决方法是在SelectedItem绑定上添加转换器：

public class IgnoreNewItemPlaceholderConverter : IValueConverter
{
    private const string newItemPlaceholderName = "{NewItemPlaceholder}";
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value != null && value.ToString() == newItemPlaceholderName)
            value = DependencyProperty.UnsetValue;
        return value;
    }
}

在XAML中使用它的一个例子是：

<Window.Resources>   
    <converters:IgnoreNewItemPlaceHolderConverter x:Key="ignoreNewItemPlaceHolderConverter"/>
</Window.Resources>
<toolkit:DataGrid ItemsSource="{Binding Persons}" 
                  AutoGenerateColumns="False"
                  SelectedItem="{Binding SelectedPerson, Converter={StaticResource ignoreNewItemPlaceHolderConverter}}"
                  IsSynchronizedWithCurrentItem="True">...</toolkit:DataGrid>

我的问题是我试图将此“修复”/“黑客”实施到我自己的DataGrid但没有成功。我有一个自定义DataGrid，其中我通过以下方式覆盖了标准DataGrid控件：

/// <summary>
/// Class that overrides the standard DataGrid and facilitates the 
/// the loading and binding of multiple cultures.
/// </summary>
public class ResourceDataGrid : DataGrid
{
    private IResourceStrategy strategy;
    protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e)
    {
        base.OnPropertyChanged(e);
        if (e.Property == DataContextProperty)
            HandleDataContextChanged(e.OldValue, e.NewValue);
        if (e.Property == ItemsSourceProperty)
            HandleItemsSourceChanged(e.OldValue, e.NewValue);
    }

    private void HandleDataContextChanged(object oldValue, object newValue)
    {
        if (strategy != null)
            strategy.ResourceCulturesChanged -= Strategy_ResourceAdded;

        // Pull in the required data from the strategy.
        var resourceDataViewModel = newValue as ResourceDataViewModel;
        if (resourceDataViewModel == null)
            return;

        strategy = resourceDataViewModel.Strategy;
        strategy.ResourceCulturesChanged += Strategy_ResourceAdded;
    }

    private void Strategy_ResourceAdded(object sender, ResourceCollectionChangedEventArgs args)
    {
        UpdateGrid();
    }

    private void HandleItemsSourceChanged(object oldValue, object newValue)
    {
        if (Equals(newValue, oldValue))
            return;
        UpdateGrid();
    }

    private void UpdateGrid()
    {
        if (strategy == null) return;

        // Update the bound data set.
        foreach (CollectionTextColumn item in Columns.OfType<CollectionTextColumn>().ToList())
        {
            // Remove dynamic columns of the current CollectionTextColumn. 
            foreach (var dynamicColumn in Columns.OfType<DynamicTextColumn>().ToList())
                Columns.Remove(dynamicColumn);

            int itemColumnIndex = Columns.IndexOf(item) + 1;
            string collectionName = item.Collection;
            List<string> headers = strategy.ResourceData.FileCultureDictionary.Select(c => c.Value).ToList();

            // Check if ItemsSource is IEnumerable<object>.
            var data = ItemsSource as IEnumerable<object>;
            if (data == null)
                return;

            // Copy to list to allow for multiple iterations.
            List<object> dataList = data.ToList();
            var collections = dataList.Select(d => GetCollection(collectionName, d));
            int maxItems = collections.Max(c => c.Count());
            for (int i = 0; i < maxItems; i++)
            {
                // Header binding.
                string header = GetHeader(headers, i);
                Binding columnBinding = new Binding(String.Format("{0}[{1}]", item.Collection, i));
                Columns.Insert(itemColumnIndex + i,
                                    new DynamicTextColumn(item) { Binding = columnBinding, Header = header });
            }
        }
    }

    private IEnumerable<object> GetCollection(string collectionName, object collectionHolder)
    {
        // Reflect the property which holds the collection.
        PropertyInfo propertyInfo = collectionHolder.GetType().GetProperty(collectionName);
        object propertyValue = propertyInfo.GetValue(collectionHolder, null);
        var collection = propertyValue as IEnumerable<object>;
        return collection;
    }

    private static string GetHeader(IList<string> headerList, int index)
    {
        int listIndex = index % headerList.Count;
        return headerList[listIndex];
    }
}

为了显示绑定，我在XAML中使用ResourceDataGrid，如下所示：

<Window.Resources>   
    <converters:IgnoreNewItemPlaceHolderConverter x:Key="ignoreNewItemPlaceHolderConverter"/>
</Window.Resources>
<Controls:ResourceDataGrid x:Name="resourceDataGrid" 
                           IsSynchronizedWithCurrentItem="True" 
                           SelectedItem="{Binding SelectedResource, Converter={StaticResource ignoreNewItemPlaceholderConverter}, Mode=TwoWay}"
                           ItemsSource="{Binding Path=Resources, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, IsAsync=True}">
    <Controls:ResourceDataGrid.Columns>
        <DataGridTemplateColumn Header="KeyIndex" SortMemberPath="KeyIndex" CellStyle="{StaticResource MetroDataGridCell}" 
                                CellTemplate="{StaticResource readOnlyCellUpdatedStyle}" IsReadOnly="True"/>
        <DataGridTextColumn Header="FileName" CellStyle="{StaticResource MetroDataGridCell}"
                            Binding="{Binding FileName}" IsReadOnly="True"/>
        <DataGridTextColumn Header="ResourceName" Binding="{Binding ResourceName}" 
                            CellStyle="{StaticResource MetroDataGridCell}" IsReadOnly="False"/>
        <Controls:CollectionTextColumn Collection="ResourceStringList" Visibility="Collapsed" 
                                       CellStyle="{StaticResource MetroDataGridCell}"/>
    </Controls:ResourceDataGrid.Columns>
</Controls:ResourceDataGrid>

现在，我实现IgnoreNewItemPlaceHolderConverter转换器并调用它并设置DependencyProperty.UnsetValue;这是它的工作。但是，已调用被覆盖的OnPropertyChanged事件且DependencyPropertyChangedEventArgs e包含验证错误：

ErrorContent =“无法转换”值'{NewItemPlaceholder}'。“

我已经实现了一个包含两列的基本示例，这是有效的。 这是由于我的更复杂的自定义DataGrid，如何阻止此验证错误发生？

感谢您的时间。

Answer 1

如果你做了mvvm，你可以采取另一种方式，如果你可行的话。我在我的项目中设置了CanUserAddRows =“False”，但是添加了一个带有ICommand“AddNewItemCommand”的按钮在这些命令中，我只需在我的ItemsSource集合中添加一个新项目 - 然后我就完成了：）

如果这对你没办法 - 只要忽略我的答案：）

Answer 2

我设法通过从拖动列表中删除元素来获得解决方法，请参阅下面的代码段：

private void preventDragEmpty(object sender, DragEventArgs e)
{
    List<dynamic> h = new List<dynamic>();
    try
    {
        //i'm using GongSolutions to handle drag and drop wich is highlly recommended
        //but if you dont use it just adapt to the correct type!
        h = e.Data.GetData("GongSolutions.Wpf.DragDrop") as List<dynamic>;

        if (h != null)
        {
            h.Remove(h.FirstOrDefault(x => x.ToString() == "{NewItemPlaceholder}"));
            e.Data.SetData(h);
        }
    }
    finally
    {
        e.Handled = true;
    }
}

要使用它，您可以附加到任何类型的列表，如：

<DataGrid ... PreviewDragOver="preventDragEmpty" />

WPF DataGrid中的错误

2 个答案: