Default value for comboBox

Hi I try set default value for comboBox.

XAML:

        <ComboBox Name="StatusCombo"
                  Style="{StaticResource statusComboStyle}"
                  SelectedValuePath="StatusMsg" 
                  SelectedValue="{Binding Path=SelectedStatus}"/>

I use caliburn.Micro. I bind List<string, StatusItem> to ComboBox, it works good.

Status item is simple class, here is it:

public class StatusItem
{
    public string StatusMsg { get; set; }
    public BitmapImage StatusImg { get; set; }
}

App.xaml

I define empty string in app.xaml

<System:String x:Key="empty"></System:String>

statusComboStyle is here:

   <Style x:Key="statusComboStyle" TargetType="{x:Type ComboBox}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path= SelectedStatus}" Value="{StaticResource empty}">
                   <Setter Property="SelectedIndex" Value="0"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>

But something must be wrong, because I get this compile error:

{"No matching constructor found on type 'System.String'. You can use the Arguments or FactoryMethod directives to construct this type."}

at System.Windows.Markup.XamlReader.RewrapException(Exception e, IXamlLineInfo lineInfo, Uri baseUri)
at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream开发者_高级运维)
at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
at Spirit.Views.LogOnView.InitializeComponent() in c:\Users\Jan\Documents\Visual Studio 2010\Projects\C#\Pokec_Messenger\ver.beta\Pokec__Messenger\Spirit_Caliburn_Micro_v1.0\Views\LogOnView.xaml:line 1
at Spirit.Views.LogOnView..ctor() in C:\Users\Jan\Documents\Visual Studio 2010\Projects\C#\Pokec_Messenger\ver.beta\Pokec__Messenger\Spirit_Caliburn_Micro_v1.0\Views\LogOnView.xaml.cs:line 24

I check in data trigger if SelectedStatus is empty string, if value is empty string I set frist item in comboBox.

You do not need to create your own empty string, string has a static field for that, so you could set it in the style like this:

Value="{x:Static System:String.Empty}"

And why don't you just use a normal setter in your style right away?

<Style x:Key="statusComboStyle" TargetType="{x:Type ComboBox}">
    <Setter Property="SelectedIndex" Value="0"/>
</Style>

(You should make sure that there is at least one item in the ComboBox)

Binding="{Binding Path=Name.Length, FallbackValue=0, TargetNullValue=0}" Value="0">