PrismLibrary/Prism · info · NotImplementedException

Not implemented

Error message

Not implemented

What it means

RelativeContentSizeConverter converts a RelativeSize to pixels for one-way sizing of dialogs; the reverse conversion has no meaningful implementation, so ConvertBack throws NotImplementedException. Two-way binding to a property using this converter is unsupported by design.

Solutions

1. Change the binding to Mode=OneWay (the only supported direction for this converter).
2. Remove the converter from any editable/target-updating binding.
3. If reverse conversion is needed, write a custom IValueConverter implementing ConvertBack.

Example fix

// before
<BindableProperty Binding="{Binding Width, Converter={StaticResource RelativeContentSize}, Mode=TwoWay}" />
// after
<BindableProperty Binding="{Binding Width, Converter={StaticResource RelativeContentSize}, Mode=OneWay}" />

// audit bindings using the converter
if (binding.Mode is BindingMode.TwoWay or BindingMode.OneWayToSource)
    throw new InvalidOperationException("RelativeContentSizeConverter only supports OneWay");

When it happens

Trigger: Binding with Mode=TwoWay (or OneWayToSource) on a property whose Binding uses RelativeContentSizeConverter, causing WPF/MAUI to call ConvertBack.

Common situations: Copy-pasting a dialog XAML snippet and changing the size binding to TwoWay; applying the converter to an editable control's value (e.g. an Entry bound to the sized property); using the converter outside Prism's relative-size scenario.

Related errors

• An error occurred while initializing the Modules.
• An error was encountered while configuring the Module…
• An error was encountered while invoking the OnInitialized…
• ArgumentNullException for 'regionTarget'
• ArgumentNullException for 'target'

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/4784f1a1de41511b. Report an issue: GitHub.

Appendix: source

Thrown at src/Maui/Prism.Maui/Dialogs/RelativeContentSizeConverter.cs:39

            else
            {
                relativeSize = value;
            }
        }
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var pageSize = double.Parse(value.ToString());
        if (pageSize < 0)
            return pageSize;

        return RelativeSize * pageSize;
    }

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

View on GitHub (pinned to 358118cd64)