title: Remove the item which is selected in other ComboBox from source?
body: <p>Suppose we have four ComboBoxes in view (<a href="https://gist.github.com/JerryBian/7804536" rel="nofollow">source</a>):</p>
  
  <pre><code>&lt;StackPanel&gt;
      &lt;ComboBox ItemsSource="{Binding SourceCollection1}" DisplayMemberPath="Name" SelectedItem="{Binding Selected1}"/&gt;
      &lt;ComboBox ItemsSource="{Binding SourceCollection2}" DisplayMemberPath="Name" SelectedItem="{Binding Selected2}"/&gt;
      &lt;ComboBox ItemsSource="{Binding SourceCollection3}" DisplayMemberPath="Name" SelectedItem="{Binding Selected3}"/&gt;
      &lt;ComboBox ItemsSource="{Binding SourceCollection4}" DisplayMemberPath="Name" SelectedItem="{Binding Selected4}"/&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p>And the VM something like (<a href="https://gist.github.com/JerryBian/7804542" rel="nofollow">source</a>):</p>
  
  <pre><code>public ObservableCollection&lt;People&gt; SourceCollection1 { get; set; }
  ...
  
  private People _selected1;
  public People Selected1
  {
      get
      {
          return _selected1;
      }
  
      set
      {
          var pc = PropertyChanged;
          if (pc != null)
          {
              _selected1 = value;
              pc(this, new PropertyChangedEventArgs("Selected1"));
          }
      }
  }
  ...
  </code></pre>
  
  <p>And the <code>People</code> class has two properties: <code>Name</code> and <code>Age</code>.</p>
  
  <p>So, I'd like to implement this feature: when user selected one item in <code>ComboBox1</code>, then the remaining ConboBoxes should adapt their own ItemsSource, removing the items which have been selected in other ComboBoxes.</p>
  
  <p>For example, the application start up, and no item has been selected yet. Then user select <code>people A</code> in <code>ComboBox1</code>, so when user opens <code>ComboBox2</code> dropdown list, the <code>people A</code> item shouldn't be include in the <code>ComboBox2</code> source. So when user select p<code>eople B</code> in <code>ComboBox2</code>, the <code>ComboBox3</code> shouldn't have <code>people A</code> and <code>people B</code>. And so on...</p>
  
  <p>Does anyone have some nice solutions? </p>