在生產環境下，經常需要監測檢視WebAPI介面的呼叫情況，日誌就成了必不可少的手段之一，本文采用NLog實現WebAPI的日誌記錄。

建立WebAPI

建立WebAPI專案，選擇ASP.NET 4.5.2模板中的WebAPI模板。

引用NLog類庫

開啟專案的NuGet包管理器，搜尋NLog，為專案新增程式包引用。

修改專案配置檔案

在webAPI專案的Web.config中進行NLog的配置。首先在節點configuration>configSections下新增節點：

此處name必需為nlog，否則配置資訊將不能被讀取。
然後在configuration節點下新增節點nlog：

這裡定義了日誌檔案的儲存路徑、命名格式以及日誌記錄型別和監聽級別。

建立日誌跟蹤相關類

建立日誌跟蹤類AppLog，繼承於System.Web.Http.Tracing下的跟蹤編寫器介面ITraceWriter，用於日誌生成和寫入：

    public sealed class AppLog : ITraceWriter
    {
        //日誌寫入
        private static readonly Logger AppLogger = LogManager.GetCurrentClassLogger();
        private
 
 static readonly Lazy<Dictionary<TraceLevel, Action<string>>> LoggingMap = new Lazy<Dictionary<TraceLevel, Action<string>>>(() => new Dictionary<TraceLevel, Action<string>>
        {
            {TraceLevel.Info,AppLogger.Info },
            {TraceLevel.Debug,AppLogger.Debug },
            {TraceLevel.Error,AppLogger.Error },
            {TraceLevel.Fatal,AppLogger.Fatal },
            {TraceLevel.Warn,AppLogger.Warn }
        });


        private
 
 Dictionary<TraceLevel, Action<string>> Logger
        {
            get { return LoggingMap.Value; }
        }
        /// <summary>
        /// 跟蹤編寫器介面實現
        /// </summary>
        /// <param name="request"></param>
        /// <param name="category"></param>
        /// <param name="level"></param>
        /// <param name="traceAction"></param>
        public void Trace(HttpRequestMessage request, string category, TraceLevel level, Action<TraceRecord> traceAction)
        {
            if (level != TraceLevel.Off)//未禁用日誌跟蹤
            {
                if (traceAction != null && traceAction.Target != null)
                {
                    category = category + Environment.NewLine + "Action Parameters : " + JsonConvert.SerializeObject(traceAction.Target);
                }
                var record = new TraceRecord(request, category, level);
                if (traceAction != null)
                {
                    traceAction(record);
                }
                //  traceAction?.Invoke(record);
                Log(record);
            }
            //throw new NotImplementedException();
        }
        /// <summary>
        /// 日誌寫入
        /// </summary>
        /// <param name="record"></param>
        private void Log(TraceRecord record)
        {
            var message = new StringBuilder();

            /**************************執行日誌****************************/

            if (!string.IsNullOrWhiteSpace(record.Message))
            {
                message.Append("").Append(record.Message + Environment.NewLine);
            }

            if (record.Request != null)
            {
                if (record.Request.Method != null)
                {
                    message.Append("Method : " + record.Request.Method + Environment.NewLine);
                }

                if (record.Request.RequestUri != null)
                {
                    message.Append("").Append("URL : " + record.Request.RequestUri + Environment.NewLine);
                }

                if (record.Request.Headers != null && record.Request.Headers.Contains("Token") && record.Request.Headers.GetValues("Token") != null && record.Request.Headers.GetValues("Token").FirstOrDefault() != null)
                {
                    message.Append("").Append("Token : " + record.Request.Headers.GetValues("Token").FirstOrDefault() + Environment.NewLine);
                }
            }

            if (!string.IsNullOrWhiteSpace(record.Category))
            {
                message.Append("").Append(record.Category);
            }

            if (!string.IsNullOrWhiteSpace(record.Operator))
            {
                message.Append(" ").Append(record.Operator).Append(" ").Append(record.Operation);
            }

            //***************************異常日誌***********************************//
            if (record.Exception != null && !string.IsNullOrWhiteSpace(record.Exception.GetBaseException().Message))
            {
                var exceptionType = record.Exception.GetType();
                message.Append(Environment.NewLine);
                message.Append("").Append("Error : " + record.Exception.GetBaseException().Message + Environment.NewLine);

            }


            //日誌寫入本地檔案
            Logger[record.Level](Convert.ToString(message) + Environment.NewLine);
        }
    }

建立日誌篩選器類LogFilterAttribute，繼承於System.Web.Http.Filters下的篩選器特性基類，用於定義日誌內容：

    public class LogFilterAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            GlobalConfiguration.Configuration.Services.Replace(typeof(ITraceWriter), new AppLog());
            var trace = GlobalConfiguration.Configuration.Services.GetTraceWriter();
            trace.Info(actionContext.Request, "Controller : " + actionContext.ControllerContext.ControllerDescriptor.ControllerType.FullName + Environment.NewLine + "Action : " + actionContext.ActionDescriptor.ActionName, "JSON", actionContext.ActionArguments);
            //base.OnActionExecuting(actionContext);
        }
    }

建立異常篩選器類AbnormalFilterAttribute，繼承於System.Web.Http.Filters下的異常篩選器類，用於異常資訊的跟蹤篩選：

    public class AbnormalFilterAttribute: ExceptionFilterAttribute
    {
        public override void OnException(HttpActionExecutedContext actionExecutedContext)
        {
            GlobalConfiguration.Configuration.Services.Replace(typeof(ITraceWriter), new AppLog());
            var trace = GlobalConfiguration.Configuration.Services.GetTraceWriter();
            trace.Error(actionExecutedContext.Request, "Controller : " + actionExecutedContext.ActionContext.ControllerContext.ControllerDescriptor.ControllerType.FullName + Environment.NewLine + "Action : " + actionExecutedContext.ActionContext.ActionDescriptor.ActionName, actionExecutedContext.Exception);
            var exceptionType = actionExecutedContext.Exception.GetType();
            if (exceptionType == typeof(ValidationException))
            {
                var resp = new HttpResponseMessage(HttpStatusCode.BadRequest) { Content = new StringContent(actionExecutedContext.Exception.Message), ReasonPhrase = "ValidationException" };
                throw new HttpResponseException(resp);
            }
            else if (exceptionType == typeof(UnauthorizedAccessException))
            {
                throw new HttpResponseException(actionExecutedContext.Request.CreateResponse(HttpStatusCode.Unauthorized));
            }
            else
            {
                throw new HttpResponseException(actionExecutedContext.Request.CreateResponse(HttpStatusCode.InternalServerError));
            }
            //base.OnException(actionExecutedContext);
        }
    }

應用NLog配置

最後需要將上述配置應用到專案中去，在pp_Data>WebApiConfig中新增如下配置：

將上述步驟完成後，執行專案，呼叫API介面進行測試，呼叫完成後就可以在站點目錄下找到日誌檔案，能夠為API介面的日常執行維護提供很大的便利。