dotnet/wpf · error · NotImplementedException

new NotImplementedException()

Error message

new NotImplementedException()

What it means

RibbonWindowSmallIconConverter is a one-way value converter: only Convert is implemented. ConvertBack always throws NotImplementedException because converting an icon ImageSource back to the source object is not meaningful for the ribbon window's small-icon binding.

Solutions

  1. Change the binding to Mode=OneWay (or OneTime) so ConvertBack is never invoked.
  2. Use a different/custom converter with a real ConvertBack if reverse conversion is needed.
  3. Write a custom converter class for your scenario rather than reusing this internal-purpose converter.

Example fix

// before
<Image Source="{Binding Icon, Converter={StaticResource RibbonWindowSmallIconConverter}, Mode=TwoWay}"/>
// after
<Image Source="{Binding Icon, Converter={StaticResource RibbonWindowSmallIconConverter}, Mode=OneWay}"/>
Defensive patterns

Strategy: try-catch

Try / catch

try { value = converter.ConvertBack(v, typeof(object), null, CultureInfo.CurrentCulture); }
catch (NotImplementedException) { value = Binding.DoNothing; }

Prevention

When it happens

Trigger: Using RibbonWindowSmallIconConverter in a TwoWay or OneWayToSource binding (Binding with Mode=TwoWay, or UpdateSourceTrigger firing on the bound property), so WPF calls ConvertBack.

Common situations: Developers reusing the converter in their own bindings (e.g. Icon bound TwoWay by default in some scenarios) instead of one-way icon display bindings.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/665e7fbe84275222. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Windows.Controls.Ribbon/Microsoft/Windows/Controls/Ribbon/Primitives/RibbonWindowSmallIconConverter.cs:43

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            ImageSource imageSource = value as ImageSource;
            ImageSource returnImageSource = null;
            if (imageSource != null)
            {
                returnImageSource = GetSmallIconImageSource(imageSource);

                if (returnImageSource == null)
                {
                    returnImageSource = imageSource;
                }
            }
            return returnImageSource;
        }

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

        #endregion

        #region Helper Methods

        private ImageSource GetSmallIconImageSource(ImageSource imageSource)
        {
#if RIBBON_IN_FRAMEWORK
            Size size = new Size(SystemParameters.SmallIconWidth, SystemParameters.SmallIconHeight);
#else
            Size size = Microsoft.Windows.Shell.SystemParameters2.Current.SmallIconSize;
#endif
            bool asGoodAsItGets = false;

            var bf = imageSource as BitmapFrame;
            if (bf != null && bf.Decoder != null && bf.Decoder.Frames != null && bf.Decoder.Frames.Count > 0)
            {

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