我正在使用 MVVM Light V3 alpha 3 编写一个 WPF 4 应用程序(使用 VS2010 RC)，我在这里遇到了一些奇怪的行为......

我有一个命令可以打开 Window ，并且那个 Window 创建了 ViewModel 等等 - 没有什么奇怪的。

在那Window我有一些RelayCommand例如:

CategoryBeenSelected = new RelayCommand(() => OnCategoryUpdate = true);

没有什么奇怪的 - 它按我预期的那样工作。

问题是我不能使用通用 RelayCommand 的 CanExecute 方法/lambda 表达式。

这个有效:

DeleteCategoryCommand = new RelayCommand<int>(DeleteCategory);

但这不是:

DeleteCategoryCommand = new RelayCommand<int>(DeleteCategory, CanDeleteCategory);

窗口没有显示。我的意思是，我单击打开窗口的按钮，应用程序被阻止，几秒钟后，窗口的 InitializeComponent方法抛出 NullReferenceException (对象引用未设置为对象的实例)

简而言之，如果我放一个 CanExecute RelayCommand<T> 上的方法, Window 拥有 无法实例化 ViewModel(带有 RelayCommand<T>)。如果我删除 CanExecute , Window出现。

这里的问题在哪里？我很困惑。

谢谢。

编辑:根据要求，这里是堆栈跟踪:

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

最佳答案

似乎 RelayCommand 会将参数值转换为泛型 T。

但是您不能将 null 转换为结构，正如异常告诉您的那样!

如果您使用可为空的结构初始化 RelayCommand，它将按预期工作!

RelayCommand<int?> or RelayCommand<Nullable<int>>

HTH

关于c# - RelayCommand<T> 上的 CanExecute 不起作用，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/2306063/