[API Proposal]: Interpolated string overloads of ILogger extensions

[stephentoub]

Background and motivation

LoggerExtensions provides a plethora of methods like:

public static void LogDebug(this ILogger logger, string? message, params object?[] args)

We strongly encourage developers to use structured logging, either by having their messages to such calls be literals, with arguments filled in from the supplied params, or by using the logger generator to have strongly-typed logging methods. However, ridiculous amounts of code continue to just do the simple thing of using interpolated strings, e.g.

_logger.LogDebug($"Value: {value}");

and to do so in a way where the calls are not guarded by IsEnabled checks for the LogLevel. That means that all of the expense associated with generating the messages are incurred even if it's then immediately thrown away.

We can address that by adding interpolated string handler overloads, which can avoid the associated costs when the relevant log level isn't enabled.

API Proposal

namespace Microsoft.Extensions.Logging;

public static class LoggerExtensions
{
    +public static void Log(this ILogger logger, LogLevel logLevel, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);
    +public static void Log(this ILogger logger, LogLevel logLevel, EventId eventId, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);
    +public static void Log(this ILogger logger, LogLevel logLevel, EventId eventId, Exception? exception, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);

    +public static void LogTrace(this ILogger logger, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);
    +public static void LogTrace(this ILogger logger, EventId eventId, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);
    +public static void LogTrace(this ILogger logger, EventId eventId, Exception? exception, [InterpolatedStringHandlerArgument(nameof(logger))] ref LogInterpolatedStringHandler message);
    ... // copy above 3 overloads for LogDebug, LogInformation, LogWarning, LogError, and LogCritical

    +[EditorBrowsable(EditorBrowsableState.Never)]
    +[InterpolatedStringHandler]
    +public ref struct LogInterpolatedStringHandler
    +{
    +    public LogInterpolatedStringHandler(int literalLength, int formattedCount, ILogger logger, out bool shouldAppend);

    +    ... // exact same set of AppendLiteral / AppendFormatted overloads as are on Debug.AssertInterpolatedStringHandler
    +}
}

API Usage

_logger.LogInformation($"Value: {value}");

Alternative Designs

No response

Risks

No response

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[stephentoub]

Hmm, just realized that we'd need to be able to pass the LogLevel into the handler's ctor, and we don't have a good way to do that. We'd currently need a dedicated handler type for each LogLevel, which is prohibitive.
cc: @333fred