C# – *clean* way to make a read-only dependency-property reflect the value of another property

cdata-bindingdependency-propertiesnetwpf

The code below is my current solution.

If it takes you a couple minutes to figure out what this code is doing, I hear ya.

This is an ugly mess if ever there was one. I would kill to see an alternative, (but don't let that discourage you from responding… :-). Oh jeez, I even abbreviated it (substantially) by removing the converter code, and I STILL feel dyslexic when I look at this code.

A great example of what I am trying to mimic would be the FrameworkElement.ActualWidth property. You know how the ActualWidth property is calculated and reassigned, whenever the Width property changes, or whenever the control is redrawn, or whenever else? ——

From the developer's perspective, it just looks like data-binding hard-at-work.
But ActualWidth is a read-only dependency-property. Does Microsoft really have to go through this gigantic trash-hole of code to make that work? Or is there a simpler way that utilizes the existing functionality of the data-binding system? 

public class foo : FrameworkElement
{
    [ValueConversion(typeof(string), typeof(int))]
    public class fooConverter : IValueConverter
    {   public object Convert(  object value, Type targetType,
                                object parameter, CultureInfo culture)
        { ... }
        public object ConvertBack(  object value, Type targetType,
                                    object parameter, CultureInfo culture)
        { ... }
    }

    private static readonly fooConverter fooConv = new fooConverter();



    private static readonly DependencyPropertyKey ReadOnlyIntPropertyKey =
        DependencyProperty.RegisterReadOnly( "ReadOnlyInt", typeof(int),
                                             typeof(foo), null);
    public int ReadOnlyInt
    {   get { return (int)GetValue(ReadOnlyIntPropertyKey.DependencyProperty); }
    }



    public static readonly DependencyProperty ReadWriteStrProperty =
        DependencyProperty.Register( "ReadWriteStr", typeof(string), typeof(foo),
                                     new PropertyMetadata(ReadWriteStr_Changed));
    public string ReadWriteStr
    {   get { return (string)GetValue(ReadWriteStrProperty); }
        set { SetValue(ReadWriteStrProperty, value); }
    }



    private static void ReadWriteStr_Changed(   DependencyObject d,
                                            DependencyPropertyChangedEventArgs e)
    {   try
        {   if (d is foo)
            {   foo f = d as foo;
                f.SetValue( ReadOnlyIntPropertyKey,
                            fooConv.Convert(f.ReadWriteStr, typeof(int), null,
                                            CultureInfo.CurrentCulture));
            }
        }
        catch { }
    }
}

Best Answer

Unfortunately, you'll need most of what you have. The IValueConverter isn't required in this case, so you could simplify it down to just:

public class foo : FrameworkElement
{
    private static readonly DependencyPropertyKey ReadOnlyIntPropertyKey =
        DependencyProperty.RegisterReadOnly( "ReadOnlyInt", typeof(int),
                                         typeof(foo), null);
    public int ReadOnlyInt
    {
       get { return (int)GetValue(ReadOnlyIntPropertyKey.DependencyProperty); }
    }

    public static readonly DependencyProperty ReadWriteStrProperty =
        DependencyProperty.Register( "ReadWriteStr", typeof(string), typeof(foo),
                                 new PropertyMetadata(ReadWriteStr_Changed));

    public string ReadWriteStr
    {
       get { return (string)GetValue(ReadWriteStrProperty); }
        set { SetValue(ReadWriteStrProperty, value); }
    }

    private static void ReadWriteStr_Changed(DependencyObject d,
                                        DependencyPropertyChangedEventArgs e)
    {
         foo f = d as foo;
         if (f != null)
         {
              int iVal;
              if (int.TryParse(f.ReadWriteStr, out iVal))
                  f.SetValue( ReadOnlyIntPropertyKey, iVal);
         }
    }
}
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