通过Application.Current.Resources更改外观/主题时出现NullReferenceException

Question

我正在编写一个包含多个皮肤的wpf应用程序。切换主题的代码如下：

try
{
    Application.Current.Resources.MergedDictionaries.Add( resource );
}
catch( Exception ex )
{

}

第一次调用代码（切换到新主题）时，它成功执行;但是，对相同代码的任何后续调用都会抛出System.NullReferenceException。

我设置主题的方式与此处描述的类似：http://www.codewrecks.com/blog/index.php/2008/05/22/simple-skinnable-and-theme-management-in-wpf-user-interface/。

之前有没有人见过这个错误？我怎么去调试这个，因为抛出异常的地方呢？任何帮助是极大的赞赏。

StackTrace:
       at System.Windows.EffectiveValueEntry.GetFlattenedEntry(RequestFlags requests)
       at System.Windows.DependencyObject.EvaluateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry newEntry, OperationType operationType)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.StyleHelper.ApplyStyleOrTemplateValue(FrameworkObject fo, DependencyProperty dp)
       at System.Windows.StyleHelper.InvalidateContainerDependents(DependencyObject container, FrugalStructList`1& exclusionContainerDependents, FrugalStructList`1& oldContainerDependents, FrugalStructList`1& newContainerDependents)
       at System.Windows.StyleHelper.DoStyleInvalidations(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle)
       at System.Windows.StyleHelper.UpdateStyleCache(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle, Style& styleCache)
       at System.Windows.FrameworkElement.OnStyleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
       at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
       at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
       at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp)
       at System.Windows.FrameworkElement.UpdateStyleProperty()
       at System.Windows.TreeWalkHelper.InvalidateStyleAndReferences(DependencyObject d, ResourcesChangeInfo info, Boolean containsTypeOfKey)
       at System.Windows.TreeWalkHelper.OnResourcesChanged(DependencyObject d, ResourcesChangeInfo info, Boolean raiseResourceChangedEvent)
       at System.Windows.TreeWalkHelper.OnResourcesChangedCallback(DependencyObject d, ResourcesChangeInfo info)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1._VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.VisitNode(FrameworkElement fe)
       at System.Windows.DescendentsWalker`1.VisitNode(DependencyObject d)
       at System.Windows.DescendentsWalker`1.WalkLogicalChildren(FrameworkElement feParent, FrameworkContentElement fceParent, IEnumerator logicalChildren)
       at System.Windows.DescendentsWalker`1.WalkFrameworkElementLogicalThenVisualChildren(FrameworkElement feParent, Boolean hasLogicalChildren)
       at System.Windows.DescendentsWalker`1.IterateChildren(DependencyObject d)
       at System.Windows.DescendentsWalker`1.StartWalk(DependencyObject startNode, Boolean skipStartNode)
       at System.Windows.TreeWalkHelper.InvalidateOnResourcesChange(FrameworkElement fe, FrameworkContentElement fce, ResourcesChangeInfo info)
       at System.Windows.Application.InvalidateResourceReferenceOnWindowCollection(WindowCollection wc, ResourcesChangeInfo info)
       at System.Windows.ResourceDictionary.NotifyOwners(ResourcesChangeInfo info)
       at System.Windows.ResourceDictionary.OnMergedDictionariesChanged(Object sender, NotifyCollectionChangedEventArgs e)
       at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
       at System.Collections.ObjectModel.ObservableCollection`1.InsertItem(Int32 index, T item)
       at System.Windows.ResourceDictionaryCollection.InsertItem(Int32 index, ResourceDictionary item)
       at System.Collections.ObjectModel.Collection`1.Add(T item)

Answer 1

在上面描述的相同链接中，您必须首先清除MergedDictionaries然后添加。如下。

try 
{ 
    Application.Current.Resources.MergedDictionaries.Clear();
    Application.Current.Resources.MergedDictionaries.Add( resource ); 
} 
catch( Exception ex ) 
{ 

}

您在后续调用中遇到错误的原因是因为字典只维护一个资源的一个键，它就像哈希表和多个键可能会产生问题，如果您只是继续添加其中定义了相同键的不同主题。

Answer 2

对我来说是application.current什么给了null值，而且没有.resources（或.mainwindow我真正想要的）