我正在使用MVVM Light V3 alpha 3编写一个WPF 4应用程序(使用VS2010 RC),并且在这里遇到了一些奇怪的行为......
我有一个打开 Window
的命令,那个Window创建ViewModel等等 - 那里没什么奇怪的 .
那个 Window
我有一些 RelayCommand
,例如:
CategoryBeenSelected = new RelayCommand(() => OnCategoryUpdate = true);
没有什么奇怪的 - 它按照我的预期工作 .
问题是我不能使用通用RelayCommand的CanExecute方法/ lambda表达式 .
这有效:
DeleteCategoryCommand = new RelayCommand<int>(DeleteCategory);
但这不是:
DeleteCategoryCommand = new RelayCommand<int>(DeleteCategory, CanDeleteCategory);
Window没有't show up. I mean, I click the button that opens the window, and the app just gets blocked and some seconds later, The Window' s InitializeComponent
方法抛出 NullReferenceException
(对象引用未设置为对象的实例)
简而言之,如果我在 RelayCommand<T>
上放置一个 CanExecute
方法,那么拥有该ViewModel(带有 RelayCommand<T>
)的 Window
就无法实例化 . 如果我删除了 CanExecute
,则会显示 Window
.
这里的问题在哪里?我糊涂了 .
谢谢 .
编辑:根据要求,这是堆栈跟踪:
A first chance exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll
at GalaSoft.MvvmLight.Command.RelayCommand`1.CanExecute(Object parameter)
at System.Windows.Controls.Primitives.ButtonBase.UpdateCanExecute()
at System.Windows.Controls.Primitives.ButtonBase.OnCommandChanged(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, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
at MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(Object inst, XamlMember property, Object value)
at MS.Internal.Xaml.Runtime.PartialTrustTolerantRuntime.SetValue(Object obj, XamlMember property, Object value)
at System.Xaml.XamlObjectWriter.Logic_ApplyPropertyValue(ObjectWriterContext ctx, XamlMember prop, Object value, Boolean onParent)
at System.Xaml.XamlObjectWriter.Logic_DoAssignmentToParentProperty(ObjectWriterContext ctx)
at System.Xaml.XamlObjectWriter.WriteEndObject()
at System.Windows.Markup.WpfXamlLoader.TransformNodes(XamlReader xamlReader, XamlObjectWriter xamlWriter, Boolean onlyLoadOneNode, Boolean skipJournaledProperties, Boolean shouldPassLineNumberInfo, IXamlLineInfo xamlLineInfo, IXamlLineInfoConsumer xamlLineInfoConsumer, XamlContextStack`1 stack, IStyleConnector styleConnector)
at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream)
at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
at ApuntaNotas.Views.CategoryEditorView.InitializeComponent() in c:\Users\Jesus\Documents\Visual Studio 2010\Projects\ApuntaNotas\ApuntaNotas\Views\CategoryEditorView.xaml:line 1
at ApuntaNotas.Views.CategoryEditorView..ctor() in C:\Users\Jesus\Documents\Visual Studio 2010\Projects\ApuntaNotas\ApuntaNotas\Views\CategoryEditorView.xaml.cs:line 18
A first chance exception of type 'System.NullReferenceException' occurred in PresentationFramework.dll
3 回答
看来RelayCommand会将参数的值转换为泛型T.
但是你不能将null转换为结构,因为异常告诉你!
如果使用可为空的结构初始化RelayCommand,它将按预期工作!
HTH
Arcturus在确定问题是什么方面是正确的,但是我不喜欢使用可空原语的解决方案 . 我个人不喜欢可以为空的原语,除非我有充分的理由使用它们 .
相反,我更改了RelayCommand的实现,如下所示:
我没有对通用的Execute方法进行相同的更改(至少目前为止),因为在这种情况下如果命令确实需要参数,我认为失败是不合理的 .
CanExecute的问题是WPF系统有时会在评估某些绑定之前调用它 . 例如:
在上面的XAML中,您注意到command参数绑定到控件的实际宽度 . 但是,在“imageScrollViewer”控件必须布局/渲染之前,WPF将在按钮的命令上调用CanExecute - 因此没有实际的宽度/高度 . 当用户单击按钮并调用Execute时,控件将被布局,以便将值发送到命令 . 如果不是 - 我认为失败是应该期待的 - 但仅当用户实际点击按钮时 .
当然我不喜欢CanExecute和Execute的不同行为,但是现在它似乎符合框架提出的限制 . 我可能会发现这会让我感到悲伤,但我一直都喜欢这种改变 .
也许,在这个时候,参数是
null
?