Converters to and from

[sonicsmooth]

I have a few questions about converters, in this case user string --> value --> formatted string
I have a user-typed text field which should be converted from a string with possible engineering notation, to an internal numerical value.
I also need to convert that internal numerical value to a properly formatted string in the UI TextBlock.

      <TextBox Text="{TemplateBinding DValue,
                      Converter={StaticResource myDistanceToStringConverter},
                      Mode=OneWayToSource,
                      ConverterParameter=um}" />

1. I'm not sure which is the "normal" direction for the converter. I think the "normal' direction (IValueConverter.Convert) is from Avalonia Property to UI, and that IValueConverter.ConvertBack is the opposite, but I'm not sure whether ConvertBack is used only in error cases or normal. It seemed to be called at startup when the user textbox was being automatically populated, and this happened even with OneWayToSource.
2. I only need the direction to be from the UI to the property in this case. Changing Mode=OneWayToSource or TwoWay didn't help matters.
3. What should I return when the value cannot be converted, such as if the user types "hello" instead of a proper number such as "15 mm". I tried returning null, new BindingNotification(new InvalidCastException(), BindingErrorType.Error), and throwing new NotSupportedException(), but these did not result in the red mark showing up in the UI.
4. The underlying property is a custom type Distance, with a question mark Distance? so it can be null if the conversion fails, but that didn't work.
5. Should I somehow try validating this? Would this validation occur before the conversion is attempted?
6. Given https://groups.google.com/g/wpf-disciples/c/P-JwzRB_GE8 should I just do the conversions "manually" in the setter and handling events? I can instantiate three properties -- UserString, DValue, OutputString. It's not as elegant I suppose.
7. This is in a TemplatedControl and all the properties are defined in codebehind, not viewmodel, if that makes any difference.

[timunie]

In a TemplatedControl, I probably would calculate the other value in code behind. You can check out ColorPicker or other controls for example:

Avalonia/src/Avalonia.Controls.ColorPicker/ColorSlider/ColorSlider.cs

Line 173 in 5896e33

private void SetColorToSliderValues()

[sonicsmooth]

thanks will check it out. I've been meaning to user the new color picker too...