Enum getting cast to string when bound with custom converter - wpf

Question

Problem Overview
I have a custom IValueConverter called EnumDisplayConverter. It's supposed to take an Enum value and return the name so it can be displayed. Somehow, even though this converter is being used on a binding between properties of an Enum type, the converter is being passed a value of String.Empty. This of course causes an error as String is not an Enum, not to mention it's just really unexpected.
Code to Reproduce
The following code can be used to reproduce the error. The steps to reproduce and an explanation of what the code is meant to do come after.
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:VBTest"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
<DockPanel>
<ListBox Name="LB_Foos" DockPanel.Dock="Left" ItemsSource="{Binding FooOptions}" SelectionChanged="ListBox_SelectionChanged"/>
<ComboBox ItemsSource="{x:Static local:MainWindow.SelectableThings}" SelectedItem="{Binding OpenFoo.SelectableChosenThing}" VerticalAlignment="Center" HorizontalAlignment="Center" Width="100">
<ComboBox.ItemTemplate>
<DataTemplate>
<ContentControl>
<ContentControl.Style>
<Style TargetType="ContentControl">
<Setter Property="Content" Value="{Binding Converter={x:Static local:EnumDisplayConverter.Instance}}"/>
<Style.Triggers>
<DataTrigger Binding="{Binding}" Value="-1">
<Setter Property="Content" Value="None"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Style>
</ContentControl>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</DockPanel>
</Window>
Imports System.Collections.ObjectModel
Imports System.Globalization
Class MainWindow
Shared Sub New()
Dim Things = (From v As Thing In [Enum].GetValues(GetType(Thing))).ToList
Things.Insert(0, -1)
SelectableThings = New ReadOnlyCollection(Of Thing)(Things)
End Sub
Public Shared ReadOnly Property SelectableThings As IReadOnlyList(Of Thing)
Public ReadOnly Property FooOptions As New ReadOnlyCollection(Of Integer)({1, 2, 3, 4})
'This is a placeholder method meant to set OpenFoo to a new instance of Foo when a selection is made.
'In the actual application, this is done with data binding and involves async database calls.
Private Sub ListBox_SelectionChanged(sender As Object, e As SelectionChangedEventArgs)
OpenFoo = Nothing
Select Case LB_Foos.SelectedItem
Case 1
OpenFoo = New Foo With {.ChosenThing = Nothing}
Case 2
OpenFoo = New Foo With {.ChosenThing = Thing.A}
Case 3
OpenFoo = New Foo With {.ChosenThing = Thing.B}
Case 4
OpenFoo = New Foo With {.ChosenThing = Thing.C}
End Select
End Sub
Public Property OpenFoo As Foo
Get
Return GetValue(OpenFooProperty)
End Get
Set(ByVal value As Foo)
SetValue(OpenFooProperty, value)
End Set
End Property
Public Shared ReadOnly OpenFooProperty As DependencyProperty =
DependencyProperty.Register("OpenFoo",
GetType(Foo), GetType(MainWindow))
End Class
Public Enum Thing
A
B
C
End Enum
Public Class Foo
Public Property ChosenThing As Thing?
Public Property SelectableChosenThing As Thing
Get
Return If(_ChosenThing, -1)
End Get
Set(value As Thing)
Dim v As Thing? = If(value = -1, New Thing?, value)
ChosenThing = v
End Set
End Property
End Class
Public Class EnumDisplayConverter
Implements IValueConverter
Public Shared ReadOnly Property Instance As New EnumDisplayConverter
Public Function Convert(value As Object, targetType As Type, parameter As Object, culture As CultureInfo) As Object Implements IValueConverter.Convert
If value Is Nothing Then Return Nothing
Return [Enum].GetName(value.GetType, value)
End Function
Public Function ConvertBack(value As Object, targetType As Type, parameter As Object, culture As CultureInfo) As Object Implements IValueConverter.ConvertBack
Return Binding.DoNothing
End Function
End Class
Steps to Reproduce
Run MainWindow
Select any item from the ListBox on the left
Select a different item from the ListBox
Observe unhanded exception
Code Explanation
In case it's not clear what the code is supposed to do, I'll explain a bit.
Foo represents a data object that is being edited by the user via MainWindow. Every Foo has the option of a Thing. Not having a Thing is also an option, which is why ChosenThing is a Thing? (i.e. Nullable(Of Thing)).
Null data items don't work in a ComboBox, since Null means "there is no selection". To get around this, I add a value of -1 to my list of selectable Thing values. In Foo.SelectableChosenThing, I check for -1 and convert it to Null for the actual value of Foo.ChosenThing. This lets me bind to the ComboBox correctly.
Problem Details
The error only seems to occur when OpenFoo is set to Nothing before being given a new value. If I take out the line OpenFoo = Nothing, everything works. However, in the real application I want to set OpenFoo to Nothing while the selection is being loaded- besides, it doesn't explain why this is happening in the first place.
Why is EnumDisplayConverter being passed a value of type String, when the properties involved are of the expected type Thing?

Answer 1

Turns out, after some investigation, that the problem comes from the ComboBox control. When the selected item of the ComboBox is null, it replaces the display value of null with String.Empty. Here's a snippet from the .NET Reference Source for ComboBox.UpdateSelectionBoxItem:
...
// display a null item by an empty string
if (item == null)
{
item = String.Empty;
itemTemplate = ContentPresenter.StringContentTemplate;
}
SelectionBoxItem = item;
SelectionBoxItemTemplate = itemTemplate;
...
This happens before any DataTemplates are considered, so by the time my DataTemplate get's called, it's being given a value of String.Empty to display instead of null.
The solution is to either
Add a DataTrigger to the ContentControl's Style which checks for String.Empty and doesn't use the converter in such a case.
Modify EnumDisplayConverter to check for non-Enum values and return DependencyProperty.UnsetValue.