WPF

Grid grid = new Grid();
grid.Children.Add(new TextBlock { Text = "Hello"});

listBox.Items.Add(grid);

<ListBox x:Name="listBox" HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <Grid HorizontalAlignment="Stretch">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>

                <TextBlock Grid.Column="0" Text="{Binding Property1}" FontWeight="{Binding Path=Property1, Converter={StaticResource fontWeightConverter}}" />
                <TextBlock Grid.Column="1" Text="{Binding Property2}" FontWeight="{Binding Path=Property2, Converter={StaticResource fontWeightConverter}}" />
                <TextBlock Grid.Column="2" Text="{Binding Property3}" FontWeight="{Binding Path=Property3, Converter={StaticResource fontWeightConverter}}" />
            </Grid>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

listBox.ItemsSource = new MyClass[] {
    new MyClass { Property1 = "a1", Property2 = "a2", Property3 = "a3"},
    new MyClass { Property1 = "b1", Property2 = "b2", Property3 = "b3"},
    new MyClass { Property1 = "c1", Property2 = "c2", Property3 = "c3"},
};

public class FontWeightConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        String item = value as String;
        if (item == null) return FontWeights.Normal;

        int index = int.Parse(item.Substring(1, 1));

        return index % 3 == 1 ? FontWeights.Bold : FontWeights.Normal;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

<WrapPanel ItemWidth="45" Width="225" />

teejayem wrote:

Making the letter property nullable will fix your paticular issue.

teejayem wrote:

I would personally use a Selector (like combobox) so the user can choose a valid Letter instead of guessing which letters are valid.