WPF ComboBox not Firing on selection - wpf

Question

I have a ComboBox, shown below. Why isn't the code-behind always firing?
XAML:
<ComboBox Height="23"
Name="cbAppendCreate"
VerticalAlignment="Top"
Width="120"
Margin="{StaticResource ConsistentMargins}"
ItemsSource="{Binding Path=CbCreateAppendItems}"
SelectedValue="{Binding Path=CbAppendCreate,UpdateSourceTrigger=PropertyChanged}" />
CodeBehind:
private string cbAppendCreate;
public string CbAppendCreate {
get {
//....
return cbAppendCreate
}
set { //This doesn't fire when selecting the first of 2 Items,
//but always fires when selecting the 2nd of two items
//....
cbAppendCreate = value;
}
}

Answer 1

I'll post my working code here, it's very simple. I've just created a default WPF app using VS2012 template. Here's MainWindow.xaml content:
<Window x:Class="
WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<StackPanel>
<ComboBox Height="23"
Name="cbAppendCreate"
VerticalAlignment="Top"
Width="120"
ItemsSource="{Binding Path=CbCreateAppendItems}"
SelectedValue="{Binding Path=CbAppendCreate,UpdateSourceTrigger=PropertyChanged}" />
<TextBlock Text="{Binding CbAppendCreate}"></TextBlock>
</StackPanel>
Here's code-behind:
namespace WpfApplication1
{
public class DataSource
{
public List<string> CbCreateAppendItems { get; set; }
public string CbAppendCreate { get; set; }
public DataSource()
{
CbCreateAppendItems = new List<string>() { "create", "append" };
}
}
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
DataContext = new DataSource();
}
}
}
When I select different values in combo-box, the TextBlock updates to the same value, hence the VM's property is also updated.