[BUG] RelayCommand with default parameter not executing

[DavidStania]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

Hi all,

I've created a new relaycommand like this here:

        private bool ResetFormCanExecute()
        {
            return !ImageLoading;
        }

        [RelayCommand(CanExecute = nameof(ResetFormCanExecute))]
        private async void ResetForm(bool alert = false)
        {
            if (alert && !(await _messageService.DisplayAlert("Reset", "Alles zurücksetzen?")))
                return;

            ScannedData = string.Empty;
            Comment = string.Empty;

            foreach (var item in Items)
            {
                System.IO.File.Delete(item.FilePath);
            }

            Items.Clear();
        }

XAML:

<Button Text="Formular zurücksetzen" 
                    Command="{Binding ResetFormCommand}" 
                    Margin="0,25,0,0" 
                    Background="{DynamicResource Gray400}"></Button>

The tooltip shows me a binding to IRelayCommand'1 but the other commands have a binding to IRelayCommand.
In my case, the Command will not execute and the canExecute method also not execute.

If I remove the parameter "bool alert = false" the relaycommand works.

Expected Behavior

Accepting a methods for RelayCommands with default parameters.

Steps To Reproduce

Create a RelayCommand with default parameters.

Link to public reproduction project repository

https://github.com/DavidStania/RelayCommandFail.git

Environment

- .NET MAUI CommunityToolkit: CommunityToolkit.Mvvm 8.1.0-preview1
- OS: Microsoft Windows 11 Pro 10.0.22621 Build 22621
- .NET MAUI: 7.0

Anything else?

No response

[Sergio0694]

This is by design. You now have a RelayCommand<bool> command (as you're wrapping ResetForm(bool), and you cannot invoke a generic command with a value type parameter with a null command parameter (it's intentionally a special case as it would otherwise lead to crashes when initializing views in many cases), see here:

dotnet/src/CommunityToolkit.Mvvm/Input/RelayCommand{T}.cs

Lines 85 to 90 in 6fff80d

	// Special case a null value for a value type argument type.
	// This ensures that no exceptions are thrown during initialization.
	if (parameter is null && default(T) is not null)
	{
	return false;
	}

You have two options to fix this:

1. Simply add the parameter in your XAML (with whatever syntax MAUI uses, I'm showing WPF/UWP/WinUI here):

<Button Command="{Binding ResetFormCommand}" CommandParameter="{x:False}"  />

2. Add a different overload with no parameter and wrap that one for the command instead.

Essentially this has to do with that parameter on its own, not with the default it's a default parameter 🙂