title: MVC MultiSelectList posts NULL when form is submitted
body: <p>I'm having difficulty capturing posted values from a MultiSelectList.  When the page loads, the select list controls get successfully populated from Lists in my model. But when the form is submitted, the model lists are null.</p>
  
  <p>These properties are in my "Contact" model.  The Contact object simply has an ExternalUserID (int) and a PublicName (string) property.</p>
  
  <pre><code>    public List&lt;Contact&gt; AvailableContacts { get; set; }
  
      public List&lt;Contact&gt; SelectedContacts { get; set; }
  </code></pre>
  
  <p>These controls are on my form.</p>
  
  <pre><code>@Html.ListBoxFor(m =&gt; m.AvailableContacts,
      new MultiSelectList(Model.AvailableContacts, "ExternalUserID", "PublicName", null),
      new { @class = "form-control", size = "15" })
  
  @Html.ListBoxFor(m =&gt; m.SelectedContacts,
      new MultiSelectList(Model.SelectedContacts, "ExternalUserID", "PublicName", null),
      new { @class = "form-control", size = "15" })
  </code></pre>
  
  <p>And this is my controller.</p>
  
  <pre><code>[HttpPost]
  [ValidateAntiForgeryToken]
  public ActionResult EditContacts(Contacts model)
  {
      if (ModelState.IsValid)
      {
          // Code to save omitted for clarity.
      }
      return View(model);
  }
  </code></pre>
  
  <p>This probably requires some simple tweak, and may have been asked before, but I've not found a solution on SO or a concise example online.  If you're inclined to flag this as a duplicate, then please include a link to a concise example.</p>
  
  <p>Thanks for your help.</p>