MainPage.xaml.cs

namespace SilverlightApplication1
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
            this.Loaded += (s, e) => { this.DataContext = new Person(); };
        }
    }

    public class Person : INotifyPropertyChanged
    {
        string NameField;
        public string Name { 
            get
            {
                return this.NameField;
            }
            set
            {
                if (value.Length < 10) throw new Exception("Erro!");
                this.NameField = value;
                if (this.PropertyChanged != null)
                    this.PropertyChanged(this, new PropertyChangedEventArgs("Name"));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}