未设置DependencyProperty引发NullReferenceException

Question

I am working on a Windows Store App. A custom class uses a DependencyProperty of an enum type. When I try to access this property without having set it before the app crashes with an NullReferenceException. The same code runs without any problem on Windows Phone.

public enum ItemDisplayType {
    None,
    Detail,
    Any,
}

public class CustomClass : UserControl {
    public CustomClass () {
        // No crash when DisplayType is set
        // DisplayType = ItemDisplayType.Any;
        this.InitializeComponent();
    }

    public static readonly DependencyProperty DisplayTypeProperty = DependencyProperty.Register("DisplayType", typeof(ItemDisplayType), typeof(CustomClass), null);
    public ItemDisplayType DisplayType{
        get { return (ItemDisplayType)GetValue(DisplayTypeProperty); }
        set {
            SetValue(DisplayTypeProperty, value);
        }
    }
}

I do not understand what's the problem here. According to the documentation an unset DependencyProperty should return the default value:

If a default value is not specified, the default value for a dependency property is null for a reference type, or the default of the type for a value type or language primitive (for example, 0 for an integer or an empty string for a string).

So what it is the problem here? Why does this code run on Windows Phone and not on Windows Store Apps?

Answer 1

You have set the default value to null at the end of your DependencyProperty declaration. Set it to ItemDisplayType.None . You may be able to use default(ItemDisplayType) , but I'm not sure if that will work.

public static readonly DependencyProperty DisplayTypeProperty = DependencyProperty.Register("DisplayType", typeof(ItemDisplayType), typeof(CustomClass), ItemDisplayType.None);