What are XAML markup extensions?

Question

I tried reading the MSDN article on markup extensions, but I can’t find out what they are (the article discusses what they do).

I cannot find a clear explanation of why we need markup extensions. If we can access a control object directly, why would we need a markup extension to access a binding object?

Do we need markup extensions so XAML is aware of the code behind (otherwise there is no way to get access any of the built in classes)? But then how can we access all the control types?

Answer 1

Markup extensions are not about access but extending functionality of markup (as the name implies) by doing whatever you want them to, like creating associations (Binding, x:Reference) or getting the type of a class (x:Type).

They can be used for just about anything, they are only necessary where the markup does not suffice on its own.

Answer 2

Rationale for markup extensions:

XAML is simple, which is a good thing. It is just an XML-based language used to declare objects and the relationships between them. One side effect of being simple is that it can be verbose. This cumbersome verbosity was one of the main reasons why the concept of markup extensions was introduced. A markup extension can be used to turn many lines of XAML into one concise expression...

Another side effect of XAML’s simplicity is that it does not have any “built in” knowledge of common artifacts used by WPF or the CLR; such as resource references, data binding, a null value, arrays, static members of a class, etc. Since XAML can be an integral part of application development there needs to be some way for developers to express those ideas in it.

<TextBox >
  <TextBox.Text>A text in TextBox</TextBox.Text>
</TextBox>

<TextBox Text="{x:Static system:Environment.UserName}" />

This latter syntax also provides a way to use values other than a literal string (i.e., which is a new object) such as an already constructed object or a static object in our assembly. In this sense markup extensions are objects which decide how a property's going to be set at runtime.