如果使用局部变量，则RelayCommand执行委托不起作用

Question

I am attempting to create an MVVM Light RelayCommand in a method:

protected RelayCommand NavigateToViewCommand(string viewName) {
#if false
    return new RelayCommand(() => {
        Debug.WriteLine("It fired.");
        Navigation.Navigate(ServiceLocator.Current.GetInstance<IViewLocator>().GetViewForNavigation("StudentPage2"));
    });
#else
    return new RelayCommand(() => {
        Debug.WriteLine("It fired.");
        Navigation.Navigate(ServiceLocator.Current.GetInstance<IViewLocator>().GetViewForNavigation(viewName));
    });
#endif
}

If I use the viewName parameter to the method in the Execute delegate for the RelayCommand , it will not fire. I am binding this command to a button. When I click the button, not even the Debug.WriteLine command fires (and a breakpoint placed on it won't break).

However, if I replace the viewName parameter with a hard-coded string that is the same as the value in viewName , the RelayCommand works fine.

Note that this code, where the command isn't used in a button executes without a problem:

void Test() {
    Command1.Execute(null);
    Command2("David").Execute(null);
}
RelayCommand Command1 { get { return new RelayCommand(() => Debug.WriteLine("cmd1 executed.")); } }
RelayCommand Command2(string msg) { return new RelayCommand(() => Debug.WriteLine("cmd2 executed: " + msg)); }

But if I bind Command2 to Button.Command in Xaml, it doesn't execute:

public ICommand TestCommand2 { get { return Command2("Cater"); } }

<Button Grid.Row="1" Grid.Column="1" Command="{Binding TestCommand2}" Content="TEST" />

Any ideas what might be going on here?

UPDATE

Further experimentation shows that using a virtual property in the Execute delegate instead of a parameter does appear to work. The command created by NavigateToViewCommand in this code works fine when bound to button.Command. That doesn't resolve the issue, of course; this is just more information.

// In base class:
protected RelayCommand NavigateToViewCommand() {
    return new RelayCommand(() => Navigation.Navigate(ServiceLocator.Current.GetInstance<IViewLocator>().GetViewForNavigation(NextPageViewName)));
}
protected virtual string NextPageViewName { get { return string.Empty; } }

// In subclass:
private ICommand m_nextPage;
public ICommand NextPageCommand { get { return m_nextPage ?? (m_nextPage = NavigateToViewCommand()); } }
protected override string NextPageViewName { get { return "StudentPage2"; } }

Answer 1

I prefered to use in-built ICommand pattern instead of this parameter passing to RelayCommand ctor , which means:

protected RelayCommand NavigateToViewCommand() 
{
    return new RelayCommand((viewName) => {
        Debug.WriteLine("It fired.");
        Navigation.Navigate(ServiceLocator.Current
                  .GetInstance<IViewLocator>()
                  .GetViewForNavigation(viewName.ToString()));
    });
}

and call execute like this:

NavigateToViewCommand().Execute("David");

It's a more gentle way to pass arguments to your command. ps.: I did not try this. I hope it has no typo and it's working fine.