LengthConverters

mainpage.xaml

<Page.Resources>
        <local:LengthConverter x:Key="lc" />
        <local:LengthBoolConverter x:Key="lb" />
    </Page.Resources>
    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <Grid.Resources>
            <local:MyViewModel x:Key="MyVM" BoundText="{Binding Path=Text, ElementName=inputbox, Mode=TwoWay}" />
        </Grid.Resources>
        <StackPanel DataContext="{Binding Source={StaticResource MyVM}}">
            <TextBox x:Name="inputbox" FontSize="24.667" Text="{Binding BoundText, Mode=TwoWay}" />
            <Button Content="test" IsEnabled="{Binding ElementName=inputbox, Path=Text.Length, Converter={StaticResource lb}}" />
            <TextBlock x:Name="outputbox" FontSize="24.667" Text="{Binding ElementName=inputbox, Path=Text.Length, Converter={StaticResource lc}}" />
            <TextBlock x:Name="vmcount" FontSize="24.667" Text="{Binding BoundText}" />
        </StackPanel>
    </Grid>

mainpage.xaml.cs

public class LengthConverter:IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, string language)
        {
            return (140 - (int)value);
        }

        public object ConvertBack(object value, Type targetType, object parameter, string language)
        {
            return value;
        }
    }

    public class LengthBoolConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, string language)
        {
            if ((int)value > 140) { return false; }
            return true;
        }

        public object ConvertBack(object value, Type targetType, object parameter, string language)
        {
            return value;
        }
    }


    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }
    }

    public class MyViewModel : DependencyObject, INotifyPropertyChanged
    {


        public string BoundText
        {
            get { return (string)GetValue(BoundTextProperty); }
            set { SetValue(BoundTextProperty, value); NotifyChanged("BoundText"); }
        }

        public static readonly DependencyProperty BoundTextProperty =
            DependencyProperty.Register("BoundText", typeof(string), typeof(MyViewModel), null);

        private int _boundLen;

        public int BoundLen
        {
            get { return _boundLen; }
            set
            {
                _boundLen = value;
                NotifyChanged("BoundLen");
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;

        public void NotifyChanged(string propertyName)
        {
            if (null != PropertyChanged)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }