Once you get in touch with the Silverlight Templated controls it became evident that there are three kind of controls you can put in the XAML. Given that the structure of the page is a hierarchical tree - for this reason is is commonly called Visual Tree - you can expand this paradigm to understand the three types of controls.
"Leaf" controls are classes inherited from Control and they terminate the hierarchy because nothing can be inserted into them
"Content" controls are inherited from the ContentControl class. These controls can host other elements inside of them. They are nodes in the hierarchy
"Items" controls are usually inherited from ItemsControl and they can contain multiple instances of child controls so they are also a particular case of nodes with many branches starting from it.
The boundary between one kind and the other is not always immediately evident when you use a control. As an example, if you consider the Button control at the first sight is may seems a leaf control but instead it is a Content control and thanks to this you can have button that hosts images and texts and not only a label.
When you start creating a templated control the choice of the class from which inheriting is really important and you have to make the choice carefully because this choice can impact the effectiveness and reusability of the control. Many times the choice of inheriting from Control is good, but you have always to try to give a chance to the ContentControl, reasoning about the final structure you want to achieve. Content control can often give an additional gear to your work paying an often low price in when you first create the control layout.
Creating a ContentControl from scratch
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The first time you create a control, it is really hard to deal with the concept of content. It seems there is nothing you can't do inheriting from the simple Control class, exposing a bunch of properties to set the various aspect of the control. For the purpose of this article we will create a Balloon control similar to the ones it is usual to find in chat clients, containing the messages exchanged between the parts. The control is basically a rectangle with a spike and at the first sight it may seem it suffice to inherit from Control and expose a property "Text" to set the content of the baloon. The solution can work fine but the first time we have to put an emoticon into the chat the things become immediately hard. Given that the "Text" property can contain a string it is very difficult to put the image of the emoticon inside of the balloon because you have to write a sort of parser to detect the emoticon inside of the text and change them to an image. The things become much more complicated if you want to support image attachments, different fonts, text styles and so on.
Read more: Silverlight Show
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