<Window x:Class="WpfApplication1.MainWindow"
        xmlns="<a href="http://schemas.microsoft.com/winfx/2006/xaml/presentation">http://schemas.microsoft.com/winfx/2006/xaml/presentation</a>"
        xmlns:x="<a href="http://schemas.microsoft.com/winfx/2006/xaml">http://schemas.microsoft.com/winfx/2006/xaml</a>"
        xmlns:wpfApplication1="clr-namespace:WpfApplication1"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <ResourceDictionary>
            <wpfApplication1:ConverterNull x:Key="ConverterNull"/>
        </ResourceDictionary>
    </Window.Resources>
    <Grid>
        
        <Grid.RowDefinitions>
            <RowDefinition Height="30"></RowDefinition>
            <RowDefinition Height="5"></RowDefinition>
            <RowDefinition Height="30"></RowDefinition>
        </Grid.RowDefinitions>

        <ComboBox Name ="cmb" DisplayMemberPath="Value" SelectedValuePath = "id" ></ComboBox>

        <TextBox Name="tb" Grid.Row="2"  Text="{Binding ElementName=cmb, Path=SelectedItem.Value,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged,Converter={StaticResource ConverterNull}}"></TextBox>
    </Grid>
</Window>

<Window.Resources>
        <ResourceDictionary>
            <wpfApplication1:ConverterNull x:Key="ConverterNull"/>
        </ResourceDictionary>
    </Window.Resources>

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Windows;
using System.Windows.Data;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        private List<Info> info;
        public MainWindow()
        {
            InitializeComponent();
            info = new List<Info>
            {
                new Info("a"),
                new Info("b"),
                new Info("c")
            };

            cmb.ItemsSource = info;
        }
    }

    public class Info
    {
        public Guid id { get; private set; }
        public string Value { get; set; }

        public Info(String val)
        {
            id = Guid.NewGuid();
            Value = val;
        }
    }

    sealed class ConverterNull : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value == null ? null : String.IsNullOrWhiteSpace(value.ToString()) ? null : value;
        }
    }
}