CODEWITHSUNDEEP
c#wpfxaml
YaSic
YaSic

Reputation: 13

Adding a UIElement child to a Panel implicitly adds it to the UIElementCollection for the Panel element

I am reading an MSDN article for learning XAML: https://learn.microsoft.com/en-us/dotnet/desktop/wpf/fundamentals/xaml?view=netdesktop-5.0#content-properties-and-collection-syntax-combined

It says the collection's element tag can be omitted, based on the XAML rules for processing collections such as IList. enter image description here

I can't find the XAML rules for processing collection that it says. I have searched for an hour to find out this rule. I can only find out information

"The Children collection of a Panel element can only consist of UIElement objects. Adding a UIElement child to a Panel implicitly adds it to the UIElementCollection for the Panel element."

in this article: https://learn.microsoft.com/en-us/dotnet/api/system.windows.controls.panel.children?view=net-5.0

Please let me know what is XAML rules for processing collection.

Upvotes: 0

Views: 642

Answers (1)

Peter Duniho
Peter Duniho

Reputation: 70671

There are three things going on here, all described by the documentation page you referenced, but in some cases elaborated elsewhere:

  1. The StackPanel.Children element may be omitted. This is because the Panel class, which StackPanel inherits, uses the [ContentProperty] attribute to indicate that the Children property is to be used when content is specified without an element tag.
  2. An element tag for the collection type itself may be omitted, because the Panel.Children property has the type UIElementCollection, which implements IList, and the XAML syntax rules specify that the XAML compiler will implicitly add items declared as content elements to the existing collection for the current element. This is documented by the Collections and collection types for XAML page. Note that in this case, the collection itself is implicitly specified, per the previous point.
  3. In fact, one may not actually specify the UIElementCollection element tag, because this would indicate the instantiation of a whole new collection object to be assigned to the Children property, but the property itself is read-only, so this is prohibited. I find the documentation's explanation (which it provides parenthetically in the passage you show in your post) misleading; it is correct to note that the class has no parameterless constructor, but there are ways in XAML around that limitation. The real problem is that even if one could construct a new instance of the collection, it can't be assigned to the Children property.

Upvotes: 1

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