Pass two UIElements as CommandParameter - wpf

Question

I have a Prism MVVM app, can easily pass one UIElement as a CommandParameter to ViewModel's Command. But now I want to pass two UIElements. Using this XAML:
<Button.CommandParameter>
<MultiBinding Converter="{StaticResource DummyMultiConverter}">
<Binding ElementName="PasswordBoxType"/>
<Binding ElementName="PasswordBoxRetype"/>
</MultiBinding>
</Button.CommandParameter>
Using this Converter:
public class DummyMultiConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
return values;
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
And a simple DelegateCommand in ViewModel:
private DelegateCommand<object> _commandCloseDialogOK;
public DelegateCommand<object> CommandCloseDialogOK =>
_commandCloseDialogOK ??
(_commandCloseDialogOK = new DelegateCommand<object>(commandParameter=> CommandCloseDialogOKExecute(commandParameter)));
public virtual void CommandCloseDialogOKExecute(object commandParameter)
{
RaiseRequestClose(new DialogResult(ButtonResult.OK));
}
public virtual bool CanExecuteCommandCloseDialogOK()
{
return true;
}
When it runs - Convert method gets values correctly, as an array of 2 PasswordBoxes. But CommandCloseDialogOKExecute gets its commandParameter parameter as an array of two nulls. Same happens if I define commandParameter as object[] instead of object. What should I do that commandParameter would be array of two PasswordBoxes?

Answer 1

First, if you are using MVVM you really should not pass view-related data, like UIElement's, to the view model.
Assuming that you know that and this really is a simplified depiction to illustrate your issue, the problem is that the framework explicitly clears the array that is passed into the Convert method before the value that Convert returns gets applied to the binding. The result is that it applies the now-empty array (nulls):
https://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Data/MultiBindingExpression.cs,1267
You can avoid this by creating another array in your Convert method:
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
return new []{values[0], values[1]};
}