Why CanExecute only get called when it's initialized in the constructor of the view model?

Question

I have a Command property, with this definition in the view model:

public RelayCommand SaveCommand => new RelayCommand(OnSave, CanSave);

whenever the ErrorsChanged of the INotifyDataErrorInfo is fired, RaiseCanExecuteChanged is called in the RelayCommand class, to enable/disable the button:

public void RaiseCanExecuteChanged()
        {
            CanExecuteChanged(this, EventArgs.Empty);
        }

the two delegates of the command are set in the constructor:

public RelayCommand(Action executeMethod, Func<bool> canExecuteMethod)
        {
            _TargetExecuteMethod = executeMethod;
            _TargetCanExecuteMethod = canExecuteMethod;
        }

but when the error state changed (when RaiseCanExecuteChanged is called) the CanSave method doesn't get called, after a while I changed the Command initialization way to be set in the constructor instead:

    public AddEditCustomerViewModel()
    {
        SaveCommand = new RelayCommand(OnSave, CanSave);
    }
    public RelayCommand SaveCommand {get;}

and it works! but why?

You say that *`CanSave` method doesnt get called* but we cannot tell when and who will call it. Also, what does `CanExecuteChanged` method do? As it stands now, its hard to figure out from the code that you have posted, what the issue could be. In other words we do not have enough info or cose to reproduce your issue. — CodingYoshi, Dec 24 '17 at 23:37

score 2 · Accepted Answer · answered Dec 25 '17 at 05:35

When you initialize the SaveCommad in the view model's constructor, it only run once when the view model is initialized, but When you define SaveCommand using:

public RelayCommand SaveCommand => new RelayCommand(OnSave, CanSave);

You are actually doing:

public RelayCommand SaveCommand {
    get {
        return new RelayCommand(OnSave, CanSave);
    }
}

In such case Whenever the getter of SaveCommand is called, it returns a new instance of RelayCommand. So when the RaiseCanExecuteChanged is called, it may not be the same object that's currently binding to the UI, hence failed to update the state

Why CanExecute only get called when it's initialized in the constructor of the view model?

1 Answers1