CustomStringInterpolationHandler.cs

using System.Runtime.CompilerServices;
......
[InterpolatedStringHandler]
public ref struct MyLoggerInterpolatedStringHandler
{
    private DefaultInterpolatedStringHandler _handler;

    public MyLoggerInterpolatedStringHandler(
        int literalLength, int formattedCount,
        MyLogger logger, out bool handlerIsValid)
    {
        if (!logger.Enabled)
        {
            _innerHandler = default;
            handlerIsValid = false;
            return;
        }

        _handler = new DefaultInterpolatedStringHandler(literalLength, formattedCount);
        handlerIsValid = true;
    }

    public void AppendLiteral(string msg)
    {
        _handler.AppendLiteral(msg);
    }

    public void AppendFormatted<T>(T msg)
    {
        _handler.AppendFormatted(msg);
    }

    public string ToStringAndClear()
    {
        return _handler.ToStringAndClear();
    }
}

LoggerTest.cs

var date = DateTime.Now;
logger.Enabled = true;  // 啟用記錄功能
logger.Log($"今天是 {date.Month} 月 {date.Day} 日");

logger.Enabled = false;  // 關閉記錄功能
logger.Log($"今天是 {date.Month} 月 {date.Day} 日");

MyLogger.cs

public class MyLogger
{
    public bool Enabled { get; set; }

    public void Log(
        [InterpolatedStringHandlerArgument("")]
        ref MyLoggerInterpolatedStringHandler handler)
    {
        if (Enabled)
        {
            string msg = handler.ToStringAndClear();
            Console.WriteLine(msg);
        }
    }
}

MyLoggerExtension.cs

public static class MyLoggerExtension
{
    public static void Log(
        this MyLogger logger, 
        [InterpolatedStringHandlerArgument("logger")] 
        ref MyLoggerInterpolatedStringHandler handler)
    {
        ......
    }
}