SelectedItem for combobox is always null - silverlight

Question

I'm working on Silverlight 4, and I have difficulties to make my combobox work correctly.
On changing selected items, selectedItem value stays null. I defined combobox as following:
<ComboBox
x:Name="ProductGroupCombobox"
Grid.Row="2"
Margin="10,15"
Height="30" Width="200"
Background="{x:Null}"
BorderBrush="{x:Null}"
ItemsSource="{Binding}"
SelectionChanged="ProductGroupCombobox_SelectionChanged"
SelectedItem="{Binding Path=ProductType, Mode=TwoWay}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=Name}" />
</DataTemplate>
<ComboBox.ItemTemplate>
</ComboBox>
Does anyone have idea?

Answer 1

Your SelectedItem property needs to be bound to an instance in your collection, whereas it appears you have your DataContext set to what I am assuming is a collection. Note how I adjusted the binding to be that of the collection and a separate property to be that of an instance within the collection.
public class MyData : INotifyPropertyChanged
{
List<String> ProductTypes {get; set;}
String _selectedProductType = String.Empty;
String SelectedProductType
{
get
{
return _selectedProductType;
}
set
{
_selectedProductType = value;
PropertyChangedEventHandler handler = PropertyChanged;
if(handler != null)
handler(this, new PropertyChangedEventArgs("SelectedProductType");
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
...
this.DataContext = new MyData();
...
<ComboBox
x:Name="ProductGroupCombobox"
Grid.Row="2"
Margin="10,15"
Height="30" Width="200"
Background="{x:Null}"
BorderBrush="{x:Null}"
ItemsSource="{Binding ProductTypes}"
SelectionChanged="ProductGroupCombobox_SelectionChanged"
SelectedItem="{Binding Path=SelectedProductType, Mode=TwoWay}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=Name}" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>