I've seen this in several XAML examples on the root XML element and I've
seen plenty of examples without this attribute. When is it needed and is
the value "root" special?
First let's talk about IDs. TheRHogue replied. I've taken his reply as the basis for the answer below with some editing...
You use the ID property for accessing an element from you code. Generally, if you don't need to access the property in code behind you don't need to set an ID.
Technically, when the element is instantiated, the variable is given a unique name or the name you declare with ID.
tb1.Text=“Hello World“; // set the textbox's text property to Hello World
If the ID property was not set, you could not access the textbox element directly.
What about the Root element?
Several XAML pages out there have set ID=”root” on the root element of the page. While this is fine to do if you want, you don't need to do it. Here is why:
All code that you embed in the page or in codebehind is compiled into a new class. This class automatically derives from the original root element you had in your page.
So in this example:
We build a new class that derives from Window and add all the page logic and some generated code into that class. When you load Page1.xaml during runtime you end up with a tree like this:
If you'd like to name that class, you can do that:
You'll then end up with:
So, in code, to refer to the root element in C# just say “this”.
this.Height = 200; //for example...
For some more details, see my January 2004 post: MarkupCompilation: XAML, BAML, .g.cs Details