using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
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.Navigation;
using System.Windows.Shapes;

namespace DataContextDemo
{
    /// <summary>
    /// Interaktionslogik für MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Person obj = new Person()
        {
            FirstName = "John",
            LastName = "Smith",
            Age = 30
        };



    public MainWindow()
    {

        InitializeComponent();



        this.DataContext = obj;
    }


}
}

<Window x:Class="DataContextDemo.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:DataContextDemo"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="80"/>
        <ColumnDefinition Width="*"/>
    </Grid.ColumnDefinitions>

    <Grid.RowDefinitions>
        <RowDefinition Height="30"/>
        <RowDefinition Height="30"/>
        <RowDefinition Height="30"/>
    </Grid.RowDefinitions>

    <TextBlock Margin="4" Text="First Name" VerticalAlignment="Center"/>
    <TextBox Margin="4" Text="{Binding Path= FirstName}" Grid.Column="1"/>

    <TextBlock Margin="4" Text="Last Name" Grid.Row="1" VerticalAlignment="Center"/>
    <TextBox Margin="4" Text="{Binding Path= LastName}" Grid.Column="2" Grid.Row="1"/>

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;

namespace DataContextDemo
{
public class Person : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public Augen Eyeobj = new Augen("Red");




    //public PropertyChangedEventHandler handler = PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }



    public string FirstName
    {
        get
        {
            return this.firstName;
        }
        set
        {
            this.firstName = value;
            NotifyPropertyChanged();
        }
    }

    public string LastName
    {
        get
        {
            return this.lastName;
        }
        set
        {
            this.lastName = value;
            NotifyPropertyChanged();
        }
    }

    public int Age
    {
        get
        {
            return this.age;
        }
        set
        {
            this.age = value;
            NotifyPropertyChanged();
        }
    }

    private string firstName { get; set; }
    public string lastName { get; set; }
    public int age { get; set; }
}
}



    <TextBlock Margin="4" Text="Age" Grid.Row="2" VerticalAlignment="Center"/>
    <TextBox Margin="4" Text="{Binding Path = Eyeobj.Farbe}" Grid.Column="3" Grid.Row="2"/>

    <TextBlock Margin="3" Text="{Binding Path=Eyeobj.Farbe}" Grid.Row="2" Grid.Column="3" x:Name="testbox"></TextBlock>

    <Button Margin="4" Grid.Row="3" Click="Button_Click"></Button>





</Grid>
</Window>

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;

namespace DataContextDemo
{
public class Augen : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public string farbe = "Blau";

    public Augen(string farbe)
    {
        Farbe = farbe;
    }


    //public PropertyChangedEventHandler handler = PropertyChanged;

    private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public string Farbe
    {
        get
        {
            return this.farbe;
        }
        set
        {
            this.farbe = value;
            NotifyPropertyChanged();
        }
    }


}
}