CanExecute on RelayCommand<T> not working

Question

I'm writing a WPF 4 app (with VS2010 RC) using MVVM Light V3 alpha 3 and am running into some weird behaviour here...

I have a command that opens a Window, and that Window creates the ViewModel and so on - nothing weird there.

In that Window I have some RelayCommands, for example:

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

Nothing weird again - it works as I expected.

The problem is that I cannot have a CanExecute method / lambda expression with a generic RelayCommand.

This works:

DeleteCategoryCommand = new RelayCommand<int>(DeleteCategory);

But this does not:

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

The Window doesn'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 method throws a NullReferenceException (Object reference not set to an instance of an object)

In short, If I put a CanExecute Method on a RelayCommand<T>, the Window that owns that ViewModel (with the RelayCommand<T>) can't be instantiated. If I remove the CanExecute, the Window shows up.

Where is the problem here? I'm confused.

Thank you.

EDIT: As requested, here is the stack trace:

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

Answer 1

It seems that the RelayCommand will cast the value the parameter to the generic T.

But you cannot cast a null to a struct, as the exception tells you!

If you initialize the RelayCommand with a nullable struct, it will work as expected!

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

HTH