I don't know how you are populating the ComboBox, but I assume it is in the XAML. In this case the SelectedItem would be a ComboBoxItem which does not implement the IConvertible interface and so cannot be converted to a double. Personally, I would use WPF's binding mechanism to implement this using 2 properties, and ObservableCollection to hold the allowed font sizes, and an int property to hold the selected font size, something like this:-
public ObservableCollection<int> Fonts
{
get
{
return new ObservableCollection<int>() { 8, 9, 10, 12, 14 };
}
}
int selectedFontSize;
public int SelectedFontSize
{
get
{
return selectedFontSize;
}
set
{
if (selectedFontSize != value)
{
selectedFontSize = value;
OnPropertyChanged("SelectedFontSize");
}
}
}
(this assumes you user control / window implements INotifyPropertyChanged). Then you can just create the bindings in your XAML like this:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition />
</Grid.RowDefinitions>
<ComboBox ItemsSource="{Binding Fonts}" SelectedItem="{Binding SelectedFontSize}" Width="200" HorizontalAlignment="Left"/>
<RichTextBox Grid.Row="1">
<FlowDocument FontSize="{Binding SelectedFontSize}">
<Paragraph >
Hello World!
</Paragraph>
</FlowDocument>
</RichTextBox>
</Grid>
and that is all there is to it, not forgetting to set the DataContext of your user control / window to itself like this in the constructor:
public MainWindow()
{
InitializeComponent();
this.DataContext = this;
}
Hope this helps
When I was a coder, we worked on algorithms. Today, we memorize APIs for countless libraries — those libraries have the algorithms - Eric Allman
