Textbox not updating CanExecute

Question

I'm working on an MVVM application that has a Save button that I'd like to disable if the Title field is empty.

Here's the code for the delegatecommand:

        _clickSaveChangesCommand = new DelegateCommand<string>(
            (s) => { saveStudentRecord(); //execute },
            (s) => { return (_student.Title != null);  /*Can execute*/ }
            );

Here's the binding:

 <TextBox Name="fldTitle" Text="{Binding Path=Student.Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="300" Height="27" />

When I update the student object from within the viewmodel, it works as expected. If, however, I create a new record and type something into the textbox, the button remains not executable. In my testing, if I try to show the value of _student.Title it shows with the value as expected.

Answer 1

You need to do something to raise the command's CanExecuteChanged event when _student.Title changes.

Are you using Prism's DelegateCommand? If so, Andy found this answer. If supported, it may be preferable to the suggestion below. But see this question, which is like your case in that the property is a "grandchild" property, not a direct property of the class that owns the command.

If you are using Prism and you can do that, do try replacing Student to see what happens. In 2016, that would have broken the command enable updating. It may still.

So if that doesn't work, this ought to.

Your DelegateCommand<T> class may have a method that does that; it's often called RaiseCanExecuteChanged() or something like that.

Likely, the best way to do this is in the setter for Student:

public Student Student
{
    get { return _student; }
    set
    {
        if (value != _student)
        {
            if (_student != null)
            {
                //  You do want to unhook this, otherwise there's a live reference 
                //  to the old _student and it won't be free to be garbage collected. 
                _student.PropertyChanged -= _student_PropertyChanged;
            }

            _student = value;

            if (_student != null)
            {
                _student.PropertyChanged += _student_PropertyChanged;
            }

            OnPropertyChanged();
        }
    }
}

private void _student_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Title")
    {
        ClickSaveChangesCommand.RaiseCanExecuteChanged();
    }
}