public partial class Window1 : Window
 {
     public Window1()
     {
         InitializeComponent();
         this.DataContext = new Person { FirstName="Rowan" };
     }
 }

 public class Person
 {
     public String FirstName { get; set; }
     public String LastName { get; set; }
 }

this.DataContext = new Person { FirstName="Rowan" };

Person a_person = new Person { FirstName="Rowan" };

public partial class Window1 : Window
 {
     Person GoodPerson{get;set;}
     public Window1()
     {
         InitializeComponent();
         GoodPerson = new Person { FirstName="Rowan" };
         this.DataContext = GoodPerson;
         GoodPerson.FirstName="Rand";
     }
 }

Person p = (Person)DataContext;