nlog配置文件的自动查找

https://github.com/NLog/NLog/blob/dev/src/NLog/Config/LoggingConfigurationFileLoader.cs#L222

 /// <summary>
        /// Get default file paths (including filename) for possible NLog config files. 
        /// </summary>
        public IEnumerable<string> GetDefaultCandidateConfigFilePaths(string filename = null)
        {
            if (filename == null)
            {
                // Scan for process specific nlog-files
                foreach (var filePath in GetAppSpecificNLogLocations())
                    yield return filePath;
            }

            // NLog.config from application directory
            string nlogConfigFile = filename ?? "NLog.config";
            string baseDirectory = PathHelpers.TrimDirectorySeparators(_appEnvironment.AppDomainBaseDirectory);
            if (!string.IsNullOrEmpty(baseDirectory))
                yield return Path.Combine(baseDirectory, nlogConfigFile);

            string nLogConfigFileLowerCase = nlogConfigFile.ToLower();
            bool platformFileSystemCaseInsensitive = nlogConfigFile == nLogConfigFileLowerCase || PlatformDetector.IsWin32;
            if (!platformFileSystemCaseInsensitive && !string.IsNullOrEmpty(baseDirectory))
                yield return Path.Combine(baseDirectory, nLogConfigFileLowerCase);

#if !NETSTANDARD1_3
            string entryAssemblyLocation = PathHelpers.TrimDirectorySeparators(_appEnvironment.EntryAssemblyLocation);
#else
            string entryAssemblyLocation = string.Empty;
#endif
            if (!string.IsNullOrEmpty(entryAssemblyLocation) && !string.Equals(entryAssemblyLocation, baseDirectory, StringComparison.OrdinalIgnoreCase))
            {
                yield return Path.Combine(entryAssemblyLocation, nlogConfigFile);
                if (!platformFileSystemCaseInsensitive)
                    yield return Path.Combine(entryAssemblyLocation, nLogConfigFileLowerCase);
            }

            if (string.IsNullOrEmpty(baseDirectory))
            {
                yield return nlogConfigFile;
                if (!platformFileSystemCaseInsensitive)
                    yield return nLogConfigFileLowerCase;
            }

            foreach (var filePath in GetPrivateBinPathNLogLocations(baseDirectory, nlogConfigFile, platformFileSystemCaseInsensitive ? nLogConfigFileLowerCase : string.Empty))
                yield return filePath;

            string nlogAssemblyLocation = filename != null ? null : LookupNLogAssemblyLocation();
            if (nlogAssemblyLocation != null)
                yield return nlogAssemblyLocation + ".nlog";
        }