摘要：Logging Application Block可以使开发人员在其应用程序中集成日志监测功能，看看随着2.0的推出给我们带来了哪些改变。
一．改进的地方
1．Logging Application Block首先带来的是名称上的改变，在1.1下它的全称应该是Logging and Instrumentation Application Block，一般把它翻译为日志和检测应用程序块，而2.0下却完全变成了日志应用程序块。
2．在1.1下，每个LogEntry只能被记录到一个Sink，而这种情况在2.0下已经不复存在，对于每个LogEntry对象，我们都可以通过Category指定很多的Sink。回忆一下在1.1时记录一个日志项：
LogEntry logEntry = new LogEntry();
logEntry.EventId = 100;
logEntry.Priority = 2;
logEntry.Message = "Informational message";
//只能设置一个
logEntry.Categorys = "UI Events";
Logger.Write(logEntry);2.0下可以添加多次：
LogEntry logEntry = new LogEntry();
logEntry.EventId = 100;
logEntry.Priority = 2;
logEntry.Message = "Informational message";
//设置多个Category
logEntry.Categories.Add("Trace");
logEntry.Categories.Add("UI Events");
Logger.Write(logEntry);
3．可以在代码中查询哪些日志项将被过滤，例如：
LogEntry logEntry = new LogEntry();
logEntry.Priority = 2;
logEntry.Categories.Add("Trace");
logEntry.Categories.Add("UI Events");
if (Logger.ShouldLog(logEntry))
{
// Event will be logged according to currently configured filters.
}
else
{
// Event will not be logged.
}
4．配置文件的改变。在1.1下关于Logging & Instrumentation Application Block的信息记录在loggingconfiguration.config文件中，2.0下所有的信息都放在了Web.config或App.config中，如：
<configuration>
<configSections>
<section name="loggingConfiguration" type="Microsoft.Practices.EnterpriseLibrary.Logging.Configuration.LoggingSettings, Microsoft.Practices.EnterpriseLibrary.Logging" />
</configSections>
<loggingConfiguration tracingEnabled="true" defaultCategory="General">
<logFilters>
<add
name="Category"
type="Microsoft.Practices.EnterpriseLibrary.Logging.Filters.CategoryFilter, Microsoft.Practices.EnterpriseLibrary.Logging"
categoryFilterMode="AllowAllExceptDenied">
<categoryFilters>
<add name="UI Events" />
</categoryFilters>
</add>
<add
name="Priority"
type="Microsoft.Practices.EnterpriseLibrary.Logging.Filters.PriorityFilter, Microsoft.Practices.EnterpriseLibrary.Logging"
minimumPriority="2"
/>
<add name="LogEnabled Filter"
type="Microsoft.Practices.EnterpriseLibrary.Logging.Filters.LogEnabledFilter, Microsoft.Practices.EnterpriseLibrary.Logging, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null"
enabled="true"
/>
</logFilters>
</loggingConfiguration>
</configuration>
二．记录日志信息
1．添加相关的引用
using Microsoft.Practices.EnterpriseLibrary.Common.Configuration;
using Microsoft.Practices.EnterpriseLibrary.Logging;
using Microsoft.Practices.EnterpriseLibrary.Logging.Configuration;
using Microsoft.Practices.EnterpriseLibrary.Logging.ExtraInformation;
using Microsoft.Practices.EnterpriseLibrary.Logging.Filters;
2．创建一个日志项
LogEntry log = new LogEntry();
log.EventId = 300;
log.Message = "Sample message";
log.Categories.Add("UI Events");
log.Severity = TraceEventType.Information;
log.Priority = 5;
3．调用Logger.Write()方法
Logger.Write(log);
三．记录日志项的扩展属性
使用基于泛型的Dictionary来记录，如下
// Create the dictionary to hold the extra information, and populate it
// with managed security information.
Dictionary<string, object> dictionary = new Dictionary<string, object>();
ManagedSecurityContextInformationProvider informationHelper = new ManagedSecurityContextInformationProvider();
informationHelper.PopulateDictionary(dictionary);
// Add a custom property for screen resolution
int width = Screen.PrimaryScreen.Bounds.Width;
int height = Screen.PrimaryScreen.Bounds.Height;
string resolution = String.Format("{0}x{1}", width, height);
dictionary.Add("Screen resolution", resolution);
// Write the log entry that contains the extra information
Logger.Write("Log entry with extra information", dictionary);
四．跟踪活动并记录上下文信息
1．调用DoDataAccess方法，完成后释放Trace对象
using (new Tracer("Trace"))
{
DoDataAccess();
}
2．创建DoDataAccess方法
private void DoDataAccess()
{
using (new Tracer("Data Access Events"))
{
// Peform work here
// Assume an error condition was detected - perform some troubleshooting.
DoTroubleShooting();
}
}
3．创建另一个方法DoTroubleShooting，并在其中创建LogEntry。
private void DoTroubleShooting()
{
string logMessage = "Simulated troubleshooting message for Logging QuickStart. " +
"Current activity=\"" + Trace.CorrelationManager.ActivityId + "\"";
LogEntry logEntry = new LogEntry();
logEntry.Categories.Clear();
logEntry.Categories.Add("Troubleshooting");
logEntry.Priority = 5;
logEntry.Severity = TraceEventType.Error;
logEntry.Message = logMessage;
Logger.Write(logEntry);
}
五．检测日志项是否被记录
创建一个日志项并设置它的信息，调用Logger.ShouldLog()方法
LogEntry logEntry = new LogEntry();
logEntry.Priority = 2;
logEntry.Categories.Add("Trace");
logEntry.Categories.Add("UI Events");
if (Logger.GetFilter<CategoryFilter>().ShouldLog(logEntry))
{
// Event will be logged according to currently configured filters.
// Perform operations (possibly expensive) to gather additional information
// for the event to be logged.
}
else
{
// Event will not be logged. Your application can avoid the performance
// penalty of collecting information for an event that will not be
// logged.
}
LogEntry logEntry = new LogEntry();
logEntry.Priority = 2;
logEntry.Categories.Add("Trace");
logEntry.Categories.Add("UI Events");
if (Logger.ShouldLog(logEntry))
{
// Event will be logged according to currently configured filters.
}
else
{
// Event will not be logged.
}
六．创建自定义的Trace Listener
1．添加特性ConfigurationElementType，需要继承自CustomTraceListener
[ConfigurationElementType(typeof(CustomTraceListenerData))]
public class DebugTraceListener : CustomTraceListener
{
//
}
2．覆写TraceData方法
public override void TraceData(TraceEventCache eventCache, string source, TraceEventType eventType, int id, object data)
{
if (data is LogEntry && this.Formatter != null)
{
this.WriteLine(this.Formatter.Format(data as LogEntry));
}
else
{
this.WriteLine(data.ToString());
}
}
3．覆写Write()和WriteLine()方法
/**//// <summary>
/// Writes a message to the debug window
/// </summary>
/// <param name="message">The string to write to the debug window</param>public override void Write(string message)
{
Debug.Write(message);
}
/**//// <summary>
/// Writes a message to the debug window
/// </summary>
/// <param name="message">The string to write to the debug window</param>public override void WriteLine(string message)
{
Debug.WriteLine(message);
}
七．创建自定义的Formatter
1．在自定义的类上添加特性ConfigurationElementType，并实现接口ILogFormatter
[ConfigurationElementType(typeof(CustomFormatterData))]
public class MyFormatter : ILogFormatter
{
//
}
2．构造函数接受一个类型为NameValueCollection的参数
public MyFormatter (NameValueCollection attributes)
{
//
}
3．添加Format方法，它接受一个LogEntry的参数
public string Format(LogEntry log)
{
//
}
八．使用场景
如果你的应用程序需要挟日志到Event Log, E-mail, Database, Message Queue, Windows Management Instrumentation (WMI), TextFile,你就应该考虑使用日志组件来提供这些功能，特别如果你需要基于分类和优先级来过滤日志消息，需要格式化消息，或者需要不改动代码的情况下改变消息的目的地。日志组件同时被设计成可扩展的，包括方便的创建客户订制的Formatter和TraceListener。
参考资料：Enterprise Libaray –January 2006帮助文档