CompositeFormat - getting the templates

[MichaelKetting]

In #50330, the original proposal containted the option of getting the list of templates parsed from the string. This would allow formatting the string using name matching. The final API proposal and implemented version (#50330 (comment)) no longer offers this information, even though it's preserved as internal information in CompositeFormat.

I looked into the discussion and information by @stephentoub but coudn't find details on why this part of the API was dropped and am hopeing on more information what I'm missing / if this is planned in the future?

Many thanks, Michael

[stephentoub]

This would allow formatting the string using name matching.

Can you elaborate on this part? CompositeFormat works with standard composite format strings, where the contents of the placeholders are numbers, not names. CompositeFormat does not support arbitrary strings like you find in logging; functionality like LoggerMessage.Define is responsible for parsing its own name-based scheme and translating that into a numeric scheme that CompositeFormat then uses.

[MichaelKetting]

Thanks for getting back so quickly on this!

My use case was to simply use named placeholders instead of indexed ones, e.g. when I have a localized text. I originally discovered this opportunity when I read the inital text of the issue, which of course, references LoggerMessage.Define.

    /// <summary>
    /// Initializes a new instance of the <see cref="CompositeFormat"/> struct.
    /// </summary>
    /// <param name="format">A template-based .NET format string as used with <c>LoggerMessage.Define</c>.</param>
    /// <param name="templates">Holds the named templates discovered in the format string.</param>
    /// <remarks>
    /// Parses a composite format string into an efficient form for later use.
    /// </remarks>
    public CompositeFormat(ReadOnlySpan<char> format, out IList<string> templates)

Anyway, if you have a localized text somewhere and you name your placeholders in the text, you can more easily reason over the localizations or use it for anything, really and don't have to document e.g. "use '0' for the hostname and '1' for the port" and I found it really cool to get this out of the box. Of course, now I see the feature didn't make the cut :)

~Michael

[DL444]

What if your placeholders must be reordered for translations? Loggers can do that because the order of placeholders are fixed and matches the order of the parameters. Not the case for localized text.

[MichaelKetting]

@DL444 My plan was do simply use the order in the list of template names to provide the args in the correct order. Since the original API idea didn't offer this out of the box, I would have done that manually.

Bascially, I saw that dotnet takes care of the heavy lifting that is parsing the format string and extracting the named placeholders (plus then creating a formatted string efficiently) and I could use that to build features.