Question

I have combobox with checkbox in data template. Combobox ItemSource property is binded with collection in ViewModel. I want to make one particular checkbox checked for default. How can I do this?

<ComboBox Grid.Column="1"
          ItemsSource="{Binding MyCollection, Mode=OneWay}" 
          Style="{StaticResource MyComboboxStyle}"
          Margin="5"
          MinWidth="120">
                <ComboBox.ItemTemplate>
                    <DataTemplate>            
                        <CheckBox Command="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=UserControl}, Path=DataContext.MyCheckedCommand}"
                                  CommandParameter="{Binding RelativeSource={RelativeSource Self}}"
                                  Content="{Binding}"
                                  IsChecked="false"
                                  VerticalAlignment="Center"
                                  Margin="3"/>                    
                    </DataTemplate>
                </ComboBox.ItemTemplate>
            </ComboBox>

Answer 1

I would make a boolean property in your viewmodel then upon loading of collection, find the object in your collection that should be checked and set if to true.

 public bool IsChecked { get; set; }

XAML:

  <CheckBox Command="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=UserControl}, Path=DataContext.MyCheckedCommand}"
                              CommandParameter="{Binding RelativeSource={RelativeSource Self}}"
                              Content="{Binding}"
                              IsChecked="{Binding IsChecked}"
                              VerticalAlignment="Center"
                              Margin="3"/>

However this may require you to have this property apart of you object model