wpf combo box default value

Question

I'm trying to default the Combo Box selected item to index = 0 when the SelectedValue is Null. What is wrong with the data trigger ? Error: SelectedIndex is not recognized property

 <ComboBox x:Name="ACombobox" ItemsSource="{Binding Mode=OneWay, Source={StaticResource AList}}" 
                    DisplayMemberPath="TypeName" SelectedValuePath="TypeName" 
                    SelectedValue="{Binding  AnObj.Type, Mode=TwoWay}"  >
                        <ComboBox.Triggers>
                            <DataTrigger Binding="{Binding}" Value="{x:Null}">
                                <Setter Property="SelectedIndex" Value="0" />
                            </DataTrigger>
                        </ComboBox.Triggers>
                    </ComboBox>

Answer 1

You should do it by creating a style trigger like below

<ComboBox x:Name="ACombobox" ItemsSource="{Binding Mode=OneWay, Source={StaticResource AList}}" 
                DisplayMemberPath="TypeName" SelectedValuePath="TypeName" 
                SelectedValue="{Binding  AnObj.Type, Mode=TwoWay}"  >
    <Style TargetType="ComboBox">
        <Style.Triggers>
            <DataTrigger Binding="{Binding}" Value="{x:Null}">
                <Setter Property="SelectedIndex" Value="0" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</ComboBox>

The error says that it does not have a qualifying type name so by creating a style it apply to Combobox when you set the TargetType="ComboBox"

---

<ComboBox x:Name="ACombobox" ItemsSource="{Binding AList}"
            DisplayMemberPath="TypeName" SelectedValuePath="TypeName" 
            SelectedValue="{Binding  AnObj.Type, Mode=TwoWay}"   >
  <ComboBox.Resources>
    <Style TargetType="ComboBox">
        <Style.Triggers>
            <DataTrigger Binding="{Binding SelectedItem}" Value="{x:Null}">
                <Setter Property="SelectedIndex" Value="0" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
  </ComboBox.Resources>
</ComboBox>

This works for me.

---

Example with StaticResource

     <Window.Resources>
        <x:Array x:Key="StringList" Type="System:String">
            <System:String>Line 1</System:String>
            <System:String>Line 2</System:String>
            <System:String>Line 3</System:String>
            <System:String>Line 4</System:String>
       </x:Array>       
    </Window.Resources>
    <ComboBox ItemsSource="{StaticResource StringList}" >
      <ComboBox.Resources>
        <Style TargetType="ComboBox">
            <Style.Triggers>
                <Trigger Property="SelectedItem" Value="{x:Null}">
                    <Setter Property="SelectedIndex" Value="0"/>
                </Trigger>
            </Style.Triggers>
        </Style>
    </ComboBox.Resources>
  </ComboBox>