title: Wpf combobox selected item not working
body: <p>I have two object: UserDto and RoleDto. User has a property which is the RoleDto.
  In my viewmodel I have the following:</p>
  
  <pre><code>public UserDto User
      {
          get { return _user; }
          set
          {
              if (_user == value) return;
  
              _user = value;
              User.PropertyChanged += UserPropertyChanged;
              OnPropertyChanged("User");
          }
      }
      private UserDto _user;
  
  public IEnumerable&lt;RoleDto&gt; Roles { get; set; } //I load all available roles in here
  </code></pre>
  
  <p>In the view, I want to select the role that the user belongs. This is how I define the combobox in the view:</p>
  
  <pre><code>&lt;ComboBox Grid.Row="3" Grid.Column="1" Margin="5" ItemsSource="{Binding Roles}" SelectedItem="{Binding User.Role, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Description" /&gt;
  </code></pre>
  
  <p>If I try to create a new user and select a role from the combobox, it is correctly binded to the user. The problem is that when I load a user that already exists, the role is not displayed in the combobox (even the user has a role defined).</p>
  
  <p>Any help please?</p>
  
  <p>Thanks in advance</p>