Adding CanExecute to RelayCommand

[minix6800]

Hi, 

First of all I would like to say a big thanks to @nice3point for the wonderfull template / samples etc

I am just looking at the "SingleProjectWpfModelessApplication" sample to better understand MVVM.

within the ViewModel, for the RelayCommand, I am trying learn how to implement CanExecute.

Lets say for the Delete button

[RelayCommand]
    private async Task DeleteElementAsync()
    {
        var deletedId = await _asyncIdExternalHandler.RaiseAsync(application =>
        {
            var document = application.ActiveUIDocument.Document;

            var selectionConfiguration = new SelectionConfiguration();
            var reference = application.ActiveUIDocument.Selection.PickObject(ObjectType.Element, selectionConfiguration.Filter);

            var transaction = new Transaction(document);
            transaction.Start("Delete element");
            document.Delete(reference.ElementId);
            transaction.Commit();

            return reference.ElementId;
        });

        TaskDialog.Show("Deleted element", $"ID: {deletedId}");
    }

lets say I want to check if someting is selected then this button can execute.

I can change the RelayCommand to 

[RelayCommand(CanExecute = nameof(IsSelectionNotNull))]

then I can create the method 

private bool IsSelectionNotNull() 
{ if (reference is null)
{return false;}
else
{return true;}
}

What I am struggling with is where to create this method while within same scope