[Proposal]: Interpolated string handler method names

[333fred]

Interpolated string handler method names

• Specification: https://github.com/dotnet/csharplang/blob/main/proposals/interpolated-string-handler-method-names.md
• Discussion: Interpolated string handler method names #9047

Summary

We add support for interpolated string handlers to receive a new piece of information, the name of the method they are an argument to, in order to solve a pain point in the creation of handler types and make them more useful in logging scenarios.

public void LogWarn(
    this ILogger logger,
    [InterpolatedStringHandlerArgument("Method Name", nameof(logger))] LogInterpolatedStringHandler message);

-------

// Original code
var someOperation = RunOperation();
ILogger logger = CreateLogger(LogLevel.Error, ...);
logger.LogWarn($"Operation was null: {operation is null}");

// Approximate translated code:
var someOperation = RunOperation();
ILogger logger = CreateLogger(LogLevel.Error, ...);
var loggingInterpolatedStringHandler = new LoggingInterpolatedStringHandler(20, 1, "LogWarn", logger, out bool continueBuilding);
if (continueBuilding)
{
    loggingInterpolatedStringHandler.AppendLiteral("Operation was null: ");
    loggingInterpolatedStringHandler.AppendFormatted(operation is null);
}
LoggingExtensions.LogWarn(logger, loggingInterpolatedStringHandler);

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-15.md#interpolated-string-handler-method-names