Howto do i get correct CommandParameter Binding with dynamic menus in wpf and mvvm

Question

i'm implementet a dynamic menu service in my WPF Application. Every Command can or should have different CommandParameters.

The Problem: With my solution to set the CommandParamter binding in xaml the CanExecute property from the command doesent update anymore.

What i have so far

I'm using mvvm-light and fody-propertychanged.

Here the menu class:

public class MyMenu : INotifyPropertyChanged
{
    private ObservableCollection<MyMenu> myChildren;

    public MyMenu()
    {
        myChildren = new ObservableCollection<MyMenu>();
    }
    public string Header { get; set; }
    public string Image { get; set; }
    public string CommandName { get; set; } //used to set the CommandParameter binding
    public ICommand Command { get; set; }
    public ObservableCollection<MyMenu> Children {
        get
        {
            return myChildren;
        }
        private set
        {
            myChildren = value;
        }
    } 


    public event PropertyChangedEventHandler PropertyChanged;
}

This class is used by the MenuService:

public sealed class MenuService : INotifyPropertyChanged
{
    private static readonly Lazy<MenuService> lazy = new Lazy<MenuService>(() => new MenuService());

    public static MenuService Instance { get { return lazy.Value; } }

    private ObservableCollection<MyMenu> myMainMenu;

    public event PropertyChangedEventHandler PropertyChanged;

    private MenuService()
    {
        myMainMenu = new ObservableCollection<MyMenu>();
    }

    public ObservableCollection<MyMenu> MainMenu
    {
        get
        {
            return myMainMenu;
        }
        private set
        {
            myMainMenu = value;
        }
    }
}

In the constructor of the viewmodel i get the instance of the MenuService and add some items:

private void AddMenuItems()
{
    MyMenu OpenUserLoginMenuItem = new MyMenu
    {
        Header = "_Login",
        Image = "./Icons/IconLogin.png",
        Command = OpenSelectTestprocedureWindowCommand,
        CommandName = "OpenUserLoginDialogCommand"
    };

    MyMenu OpenSelectTestprocedureMenuItem = new MyMenu
    {
        Header = "_Select Testprocedure",
        Image = "./Icons/IconSelectTestprocedure.png",
        Command = OpenSelectTestprocedureWindowCommand,
        CommandName = "OpenSelectTestprocedureWindowCommand"
    };

    MainMenu.Add(OpenUserLoginMenuItem);
    MainMenu.Add(OpenSelectTestprocedureMenuItem);
}

Then i have a bindable property in the viewmodel:

public ObservableCollection<MyMenu> MainMenu
{
    get
    {
        return myMenuService.MainMenu;
    }
}

Here the command implementation as RelayCommand:

//in the constructor
OpenSelectTestprocedureWindowCommand = new RelayCommand<ShowTestschrittViewParameter>(OpenSelectTestablaufWindow, CanOpenSelectTestablaufWindow);
OpenUserLoginDialogCommand = new RelayCommand<Type>(OpenUserLoginDialog);

private void OpenUserLoginDialog(Type aWindowType)
{
    myNavigationService.ShowWindowModal(aWindowType);
}

private bool CanOpenSelectTestablaufWindow(ShowTestschrittViewParameter showTestschrittViewParameter)
{
    if (myDataService.CurrentTestProcedure != null)
    {
        if (myDataService.CurrentTestProcedure.TestProcedureState == Logic.Model.GlobalTypes.TestProcedureState.Running) return false;
    }
    return new ViewModelLocator().UserLoginDialogViewModel.User.NameIsValid;
}

private void OpenSelectTestablaufWindow(ShowTestschrittViewParameter showTestschrittViewParameter)
{
    myNavigationService.ShowTestschrittView(showTestschrittViewParameter);
}

Then in the MainView i have the following xaml:

<Menu Grid.Row="2" ItemsSource="{Binding MainMenu}" Name="DynamicMenu">
    <!--<Menu.ItemTemplate>
        <DataTemplate DataType="{x:Type luih:MyMenu}">
            <StackPanel>
                    <Label Content="{Binding Header}"/>
                    <Image Source="{Binding Image}"/>
                </StackPanel>
        </DataTemplate>
    </Menu.ItemTemplate>-->
    <Menu.ItemContainerStyle>
        <Style TargetType="{x:Type MenuItem}">
            <Setter Property="Header" Value="{Binding Header}"/>
            <Setter Property="Command" Value="{Binding Command}"/>
            <Setter Property="ItemsSource" Value="{Binding Children}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding CommandName}" Value="OpenUserLoginDialogCommand">
                    <Setter Property="CommandParameter" Value="{x:Type local:UserLoginDialog}"/>
                </DataTrigger>
                <DataTrigger Binding="{Binding CommandName}" Value="OpenSelectTestprocedureWindowCommand">
                    <Setter Property="CommandParameter" Value="{x:Type local:UserLoginDialog}"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Menu.ItemContainerStyle>
</Menu>

Attention. The CommandParameter binding type in the xaml is currently not correct. This is another problem, i will solve by my self. But for testing purposes it should work. It will give me an exception because of wrong type.

But when i do the CommandParameter binding in the Style.Trigger with the DataTrigger, the CanExecute property doesent update anymore at runtime. When i'm comment this section out, everything works fine. But then i have no CommandParameters.

Any help and suggestions are welcome.

Answer 1

i'm found the problem.

RelayCommand from mvvm light evaluetes the type of the parameter for the CanExecute function. This must be the correct declared type or null.

So for testing purposes i had to set the binding like so:

 <Style TargetType="{x:Type MenuItem}">
    <Setter Property="CommandParameter" Value="{x:Null}"/>
</Style>