Question

I’m working on a WPF custom control. The control has a property that is set in code behind and used in XAML. This property must be public for it work on XAML via a Binding. Why is this, if there is just one class?

<TextBlock Text="{Binding ElementName=PolicyBoxName, Path=FileNames[0]}" />

private string[] _fileNames;
        public string[] FileNames
        {
            get
            {
                return _fileNames;
            }
            set
            {
                if (value != _fileNames)
                {
                    _fileNames = value;
                    OnPropertyChanged("FileNames");
                }
            }
        }

Answer 1

The XAML parsers constructs objects based on the supplied XML and sets their properties. It is no different from any other class, from a different namespace, that might wish to create your user control and set its properties. Without reflection, the constraints of the C# language demand that these properties are public for them to be set.