How FormattedLogValues is working ?

[WernerMairl]

Hi
i see a unexpected behavior, not sure if i'm doing something wrong....
(using .net Core 2.0.0 and packages in version 2.0.0)

Assuming that the following extension method is working like string.Format!

public static void LogTrace(this ILogger logger, string message, params object[] args)
the following sample should return 2 idenical trace strings!

string s = string.Format("{1} Bytes Markdown written to '{0}'", fn, markdown.Length);
this.Logger.LogTrace(s);
this.Logger.LogTrace("{1} Bytes Markdown written to '{0}'", fn, markdown.Length);

returns the following output on the Console-Logger (different strings)

trce: XXX[0]
352 Bytes Markdown written to 'c:\xxx.md'
trce: XXX[0]
c:\xxx.md Bytes Markdown written to '352'

This is a known issue/behavior ?
assuming that the problem comes from using format placeholder {0},{1} ordered in DESC not ASC
"{1} Bytess Markdown written to '{0}'"

internally the string calculation is done in the class FormattedLogValues, so i'm assuming that the behavior comes from there....

regards
Werner

[davidfowl]

See https://messagetemplates.org/. Those aren't regular format strings. They are message templates. You should be using meaningful names in there.

[WernerMairl]

David,
that's a valid explanation but then i suggest to change comments on the Arguments => Usability!

        /// <summary>
        /// Formats and writes a trace log message.
        /// </summary>
        /// <param name="logger">The <see cref="ILogger"/> to write to.</param>
        /// <param name="message">Format string of the log message.</param>
        /// <param name="args">An object array that contains zero or more objects to format.</param>
        public static void LogTrace(this ILogger logger, string message, params object[] args)

As a "very old" C# coder, everything in this Method declaration lets me assume that i can use string.format syntax!
The parameter "message" is commented with Format string

so please document (xml comments, Intellisense etc) that a different syntax then string.format must be used!!

it costs me a full day to find out why my tracing is producing wrong output....

regards
Werner

[davidfowl]

@WernerMairl Sorry 😞 we'll improve both the documentation and the doc comments here.

/cc @pakrym @muratg @Rick-Anderson

[pakrym]

Updated doc comments in #702, doc changes tracked in dotnet/AspNetCore.Docs#4093