In XAML, there are at least three ways to specify the contents of a collection-typed dependency property. For concreteness, I’m going to discuss UWP XAML, but the same principle apply to WPF XAML.<\/p>\n
Let’s say that we have a Doodad object with a Widgets property. The Widgets property is a collection of Widget objects.<\/p>\n
First, we have the implicit collection:<\/p>\n
<Doodad>\r\n <Doodad.Widgets>\r\n <Widget ...\/>\r\n <Widget ...\/>\r\n <Widget ...\/>\r\n <\/Doodad.Widgets>\r\n<\/Doodad>\r\n<\/pre>\nSecond, we have the explicit collection:<\/p>\n
<Doodad>\r\n <Doodad.Widgets>\r\n <MyWidgetCollection>\r\n <Widget ...\/>\r\n <Widget ...\/>\r\n <Widget ...\/>\r\n <\/MyWidgetCollection>\r\n <\/Doodad.Widgets>\r\n<\/Doodad>\r\n<\/pre>\nThird, we have binding:<\/p>\n
<Doodad Widgets=\"{Binding MyWidgets}\" ...\/>\r\n<Doodad Widgets=\"{x:Bind MyWidgets}\" ...\/>\r\n<\/pre>\nOkay, let’s tackle these in order.<\/p>\n
The XAML compiler converts the implicit collection into C# code that is roughly equivalent to the below:<\/p>\n
var e1 = new Doodad();\r\nvar widgets = e1.Widgets;\r\nwidgets.Add(new Widget(...));\r\nwidgets.Add(new Widget(...));\r\nwidgets.Add(new Widget(...));\r\n<\/pre>\n(You can ask the XAML compiler to generate C++, but I’ll use C# for notational convenience.)<\/p>\n
In order for the implicit collection to work, the property must have an initial value that is an empty collection.<\/p>\n
The explicit collection compiles to something like this:<\/p>\n
var e1 = new Doodad();\r\nvar widgets = new MyWidgetCollection();\r\nwidgets.Add(new Widget(...));\r\nwidgets.Add(new Widget(...));\r\nwidgets.Add(new Widget(...));\r\ne1.Widgets = widgets;\r\n<\/pre>\nIn order for explicit collections to work, the property must be settable.<\/p>\n
Binding operates like this:<\/p>\n
var e1 = FindTheDoodad();\r\ne1.Widgets = this.MyWidgets;\r\n<\/pre>\nThis is equivalent to the explicit collection, so that same solution for explicit collections works for binding, too.<\/p>\n
Okay, so how do we set up the dependency property so it satisfies all these requirements?<\/p>\n
Turns out this is a special case of what we looked at last time: The dependency property whose initial value is a mutable object. In this case, the mutable object is itself a collection.<\/p>\n
public class Doodad\r\n{\r\n public static readonly DependencyProperty WidgetsProperty =\r\n DependencyProperty.Register(\r\n \"Widgets\",\r\n typeof(IList<Widget>),\r\n typeof(Doodad),\r\n new PropertyMetadata(null));\r\n\r\n public IList<Widget> Widgets\r\n {\r\n get => (IList<Widget>)GetValue(WidgetsProperty);\r\n set => SetValue(WidgetsProperty, value);\r\n }\r\n\r\n public Doodad()\r\n {\r\n this.InitializeComponent();\r\n Widgets = new List<Widgets>();\r\n }\r\n ..\r\n}\r\n<\/pre>\n