Assign your own class to DataContent and bind to TextBox


   
  

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="BindingWithFallback" Height="300" Width="300">
    <StackPanel>
      <TextBox Text="{Binding Path=FastProperty, Mode=OneWay}" IsReadOnly="True" />
    </StackPanel>
</Window>

//File:Window.xaml.cs
using System;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;


namespace WpfApplication1{

  class MyClass {
    public string FastProperty { get { return "AAA"; } }
  }

  public partial class Window1 : System.Windows.Window {

    public Window1() {
      InitializeComponent();
      DataContext = new MyClass();
    }

  }
}