If you look at the code in the op you see that I trying to determine what the width of the combo needs to be based on the items in the drop down.

As you are using the ItemsSource property, the items will not be ComboBoxItems. They will be if you add them manually in the Items collection.

The foreach will throw an exception as the item cannot be caster correctly. It is probably a string or whatever the ChecklistFieldlistnameValue generic type collection is.

Here is my block of code:

XamComboEditor ListEditor = new XamComboEditor();
ListEditor.Padding = new Thickness(0);
ListEditor.HorizontalAlignment = HorizontalAlignment.Left;
ListEditor.SelectedItemChanged += new RoutedPropertyChangedEventHandler<object>(ListEditor_SelectedItemChanged);

ListEditor.DisplayMemberPath = "Description";
ListEditor.ValuePath = "StatusValue";
ListEditor.ItemsSource = fieldInfo.ChecklistFieldListName.ChecklistFieldListNameValue;

EBL.ChecklistFieldValue FieldValue = fieldInfo.ChecklistFieldValue.Where(c => c.TravelerChecklist.Id == _TravelerChecklistId).FirstOrDefault();

Binding MyBinding = new Binding("FieldValue");
MyBinding.Source = FieldValue;
MyBinding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
ListEditor.SetBinding(XamComboEditor.ValueProperty, MyBinding);

 foreach (ComboBoxItem Item in ListEditor.ComboBox.Items)
 {
  Item.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
  if (Item.DesiredSize.Width > Width) Width = Item.DesiredSize.Width;
 }

 ListEditor.Measure(new Size(double.PositiveInfinity, double.PositiveInfinity));
 ListEditor.Width = ListEditor.DesiredSize.Width + Width;

Grid.SetColumn(ListEditor, 1);
gridControls.Children.Add(ListEditor);

I am seeing what you are trying to do, but I am not seeing how you have bound the XamComboEditor. Can you please give us that code?