Question

I am facing some troubles creating a Silverlight custom control – a combobox with checkboxes.
Do we have something out of the box to do this?

Answer 1

(sorry for my bad english)

If you want to display checkboxes as items of a comboBox, a simple DataTemplate shoud work:

        <ComboBox Name="combo" >
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <CheckBox Content="{Binding Text}" IsChecked="{Binding IsChecked,Mode=TwoWay}"/>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>

Now on your code behind create an ObservableCollection<CheckItem> ChekItem is a basic class:

public class CheckItem
{
    public string Text { get; set; }
    public bool IsChecked { get; set; }
}

and associate it to your comboBox ItemsSource.
If you want to know what items are checked, is simple:

var checkedItems = myList.Where(i => i.IsChecked == true);