c# - 在设计时未设置WPF转换器抛出对象引用

我的xaml在设计时出错：
对象未设置为对象的实例。
它只在设计时发生，在运行时运行得很好。错误来自我的转换器，下面是我的代码。我想这可能是因为没有检查值是否为空，或者如果为空，我让它返回了空，但我同时更改了这两个值，它没有任何区别。
思想是值得欣赏的。谢谢

public class CouponDataSplitterConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter,     CultureInfo culture)
    {
        if (value != null)
        {
            List<SelectionArea> selectionAreasList =
                new List<SelectionArea>((ObservableCollection<SelectionArea>) value);
            foreach (var area in selectionAreasList)
            {
                if (area.AreaName.Contains(parameter.ToString()))
                {
                    return area.SelectionRows;
                }
            }

            return selectionAreasList;
        }
        return "";
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

xaml:

ItemsSource="{Binding Coupon.SelectionAreas,
        ConverterParameter='Test Parameter',
        Converter={StaticResource CouponDataSplitterConverter}

堆栈跟踪：

at App.UI.Converters.CouponDataSplitterConverter.Convert(Object      value, Type targetType, Object parameter, CultureInfo culture)
at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.Activate(Object item)
at System.Windows.Data.BindingExpression.AttachToContext(AttachAttempt attempt)
at System.Windows.Data.BindingExpression.AttachOverride(DependencyObject target, DependencyProperty dp)
at System.Windows.Data.BindingExpressionBase.OnAttach(DependencyObject d, DependencyProperty dp)
at System.Windows.StyleHelper.GetInstanceValue(UncommonField`1 dataField, DependencyObject container, FrameworkElement feChild, FrameworkContentElement fceChild, Int32 childIndex, DependencyProperty dp, Int32 i, EffectiveValueEntry& entry)
at System.Windows.StyleHelper.GetChildValueHelper(UncommonField`1 dataField, ItemStructList`1& valueLookupList, DependencyProperty dp, DependencyObject container, FrameworkObject child, Int32 childIndex, Boolean styleLookup, EffectiveValueEntry& entry, ValueLookupType& sourceType, FrameworkElementFactory templateRoot)
at System.Windows.StyleHelper.GetChildValue(UncommonField`1 dataField, DependencyObject container, Int32 childIndex, FrameworkObject child, DependencyProperty dp, FrugalStructList`1& childRecordFromChildIndex, EffectiveValueEntry& entry, ValueLookupType& sourceType, FrameworkElementFactory templateRoot)
at System.Windows.StyleHelper.GetValueFromTemplatedParent(DependencyObject  container, Int32 childIndex, FrameworkObject child, DependencyProperty dp,  FrugalStructList`1& childRecordFromChildIndex, FrameworkElementFactory templateRoot, EffectiveValueEntry& entry)
at System.Windows.StyleHelper.ApplyTemplatedParentValue(DependencyObject container, FrameworkObject child, Int32 childIndex, FrugalStructList`1& childRecordFromChildIndex, DependencyProperty dp, FrameworkElementFactory templateRoot)
at System.Windows.StyleHelper.InvalidatePropertiesOnTemplateNode(DependencyObject container, FrameworkObject child, Int32 childIndex, FrugalStructList`1& childRecordFromChildIndex, Boolean isDetach, FrameworkElementFactory templateRoot)
at System.Windows.FrameworkTemplate.InvalidatePropertiesOnTemplate(DependencyObject container, Object currentObject)
at System.Windows.FrameworkTemplate.HandleBeforeProperties(Object createdObject, DependencyObject& rootObject, DependencyObject container, FrameworkElement feContainer, INameScope nameScope)
at System.Windows.FrameworkTemplate.<>c__DisplayClass0.<LoadOptimizedTemplateContent>b__5(Object sender, XamlObjectEventArgs args)
at System.Xaml.XamlObjectWriter.OnBeforeProperties(Object value)
at System.Xaml.XamlObjectWriter.Logic_CreateAndAssignToParentStart(ObjectWriterContext ctx)
at System.Xaml.XamlObjectWriter.WriteStartMember(XamlMember property)
at System.Xaml.XamlWriter.WriteNode(XamlReader reader)
at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlReader templateReader, XamlObjectWriter currentWriter)
at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlObjectWriter objectWriter)
at System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(DependencyObject container, IComponentConnector componentConnector, IStyleConnector styleConnector, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
at System.Windows.FrameworkTemplate.LoadContent(DependencyObject container, List`1 affectedChildren)
at System.Windows.StyleHelper.ApplyTemplateContent(UncommonField`1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
at System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField`1 templateDataField, FrameworkElement container)
at System.Windows.FrameworkElement.ApplyTemplate()
at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
at System.Windows.UIElement.Measure(Size availableSize)
at System.Windows.ContextLayoutManager.UpdateLayout()
at System.Windows.UIElement.UpdateLayout()

最佳答案

如果您得到了一个nullreferenceexception，那么有些东西是空的。
代码显式检查value参数是否为空。但是，您的foreach中有这行代码：

if (area.AreaName.Contains(parameter.ToString()))

如果AreaName属性有空值（方法调用将引发Contains异常），或者提供给方法的parameter参数为空（方法调用将引发ToString异常），则此行可能引发nullreferenceexception。
在方法中放入一些调试代码可能是值得的，这样您就可以检查正在使用的变量的状态。我知道您不能使用调试器，因为这是一个设计时问题，所以您可能必须登录到临时文本文件或类似的文件。