What happens with XAML code on a build process?

When you build your project there are many things happens to the Xaml markup. For e.g.
The .g.cs file should be generated from xaml pages where you've the proper build action set as 'Page' or something valid.

Basically Microsoft.Winfx.targets is used against your xaml, and there are two build tasks getting invoked. 

1) Markupcompilepass1 http://msdn.microsoft.com/en-us/library/bb397829.aspx which generates the .g.cs file 2) Markeupcompilepass2 which creates the baml files - http://msdn.microsoft.com/en-us/library/bb397738.aspx