title: DataGrid 'EditItem' is not allowed for this view when dragging multiple items
body: <p>I have a datagrid which gets data like this:  </p>
  
  <pre><code>    public struct MyData
      {
          public string name { set; get; }
          public string artist { set; get; }
          public string location { set; get; }
      }
  
      DataGridTextColumn col1 = new DataGridTextColumn();
      col4.Binding = new Binding("name");
      dataGrid1.Columns.Add(col1);
      dataGrid1.Items.Add((new MyData() { name = "Song1", artist = "MyName", location =     "loc"}));
      dataGrid1.Items.Add((new MyData() { name = "Song2", artist = "MyName", location =     "loc2"}));
  </code></pre>
  
  <p>The problem is- whenever a user tries to edit a cell or drags multiple cells- the app throws an exception:</p>
  
  <blockquote>
    <p>System.InvalidOperationException was unhandled<br>
    Message: 'EditItem' is not allowed for this view.</p>
  </blockquote>
  
  <p>Why is this? Is it because of the way the data is entered?<br>
  Any ideas?<br>
  Thanks!</p>