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>