title: Using IEnumerable.Except
body: <p>I got 3 <code>listViews</code> 2 <code>textbox</code> and 2 <code>buttons</code> in WinForm.</p>
  
  <p>Program Description: The program adds numbers to the <code>listview</code> by typing in numbers in the <code>textbox</code> and clicking the add <code>button</code></p>
  
  <p>Goal: I want to be able to use the <code>IEnumerable.Except</code> method to output only the unique numbers in <code>listView3</code>, for example in the picture below the unique numbers are 3 and 7 in <code>listView1</code> and <code>listView2</code>.
      ListViewItem lvi = new ListViewItem(textBox1.Text);
      listView1.Items.Add(lvi);</p>
  
  <pre><code>ListViewItem lv = new ListViewItem(textBox2.Text);
  listView2.Items.Add(lv);
  
  //im doing somthing wrong here...
  var nonintersect = listView1.Except(listView2).Union(listView2.Except(listView1));
  
  //populate listview3 with the unique numbers...
  // foreach (item )
  // {
  
  // }
  </code></pre>
  
  <p>Error Message: System.Windows.Forms.ListView' does not contain a definition for 'Except' and no extension method 'Except' accepting a first argument of type 'System.Windows.Forms.ListView' could be found (are you missing a using directive or an assembly reference?)   </p>
  
  <p><a href="https://i.stack.imgur.com/neb5O.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/neb5O.png" alt="enter image description here"></a></p>