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Trace Listeners

Trace listeners are thread-safe classes that derive from an abstract class called, appropriately enough, System.Diagnostics.TraceListener. This class contains the necessary methods to be able to control trace output. There are three out-of-the-box trace listeners included with .NET. The first is called DefaultTraceListener and, by default, is automatically added to a trace listener collection shared by the application. Using the Trace.Listeners static property, you can access this collection at any time during execution. Using the collection, you can then add or remove any trace listener. Controlling the listeners in the collection will, therefore, control your trace output. When Trace.Write or Trace.WriteLine is called, these methods will emit the message to whatever trace listeners have been added to the system. The DefaultTraceListener will receive these calls and send the output to the its designated target. For the DefaultTraceListener, that target is both the OutputDebugString API and the .NET log method. You should already be familiar with the OutputDebugString API; if not, please reference the Platform SDK documentation. The log method will post the message to any attached debugger in .NET.

The second trace listener is called EventLogTraceListener. This listeners send all output to the Windows event log. Using its EventLog property, you can control which event log receives the output. The third out-of-the-box listener is called TextWriterListener. This sends all output to any TextWriter or Stream object. This can be the console's standard output stream or any file.

To add or remove a listener to or from the collection, you can either use a configuration file, such as web.config, or do it in code. The following is an example of adding or removing a listener using a configuration file:

Listing 2.11 Adding a listener using web.config.

    . . .
    <trace autoflush="true" indentsize="4">
        <add name="LogFileListener"
          initializeData="c:\LogFileListener.log" />

You can also control the listener collection in code such as the following InitTraceListeners method I use in the upcoming implementation pattern.


In the following method, I show how you can add an event log listener to the collection. However, I do not recommend you use the event log during most tracing scenarios. I typically use the event log only during error handling or more determined forms of tracing, such as message coming from Windows Services (e.g., “service starting…”, “service stopping…”, etc.). Otherwise, you will quickly fill it up if you're not careful.

Listing 2.12 Sample for adding an event log listener to a global collection.

/// <summary>
/// Adds all default trace listeners, for event log
/// tracing it first checks
/// to see if the trace level has not been set to verbose or
/// information since we
/// don't want to fill up the event viewer with verbose
/// information.
/// </summary>
public static void InitTraceListeners()
      FileStream oTextWriter = null;

      // We do not want to dump to the if tracing is set to
      // information or verbose
      if (!Config.TraceLevel.TraceInfo)
            if (Trace.Listeners[TRACE_EVENTLOG_KEY] == null)
                  EventLogTraceListener oEvtLogListener = new
                  oEvtLogListener.Name = TRACE_EVENTLOG_KEY;
      else // travel level is set to warning or error
            if (Trace.Listeners[TRACE_EVENTLOG_KEY] != null)
                  Trace Listeners.Remove(TRACE_EVENTLOG_KEY);
      if (Trace.Listeners[TRACE_TEXTWRITER_KEY] == null)
            oTextWriter = File.Exists(TRACE_LOG_FILE)) ?
                          File.OpenWrite(TRACE_LOG_FILE) :

      // This is a custom trace listener (see PMRemoteTrace.cs)
      // for remote tracing
      if (Trace.Listeners[TRACE_REMOTE_KEY] == null)

First, notice that the listener collection is manipulated like any other collection in .NET. Using .NET indexers, I can check to see whether the listener has already been added to the collection. If you look carefully at the InitTraceListeners example, you'll notice a new listener class called RemoteTrace. This is referred to as a custom trace listener and is the focus of the following implementation pattern.

By adding trace listeners to a central collection, you can globally control how and where tracing output is sent. As a developer of the rest of the system, you are required to add Trace.Write or WriteLine methods only to send the appropriate information to that output for any debug or release build. This is a powerful monitoring and logging feature but how do we elegantly control whether we want to trace at all? For that matter, how to we control at what level we would like to trace? We may simply want to trace errors or we may want to provide as much information during runtime execution as possible to help determine our problems. This is where .NET switches come into play.

Boolean and Trace Switches

The first way to turn off any tracing is by disabling the /d:TRACE option, as mentioned above. However, doing so requires the recompilation of your code for it to become effective. What if you are in production and you do not have that option? Fortunately, you can easily control tracing dynamically and without recompilation. To do so, you use what are called System.Diagnostics.Switch objects. The switch objects available in the FCL are BooleanSwitch and TraceSwitch. Both derive from System.Diagnostics.Switch. All switches are configuration objects, of sorts, that read a configuration setting and provide properties with which you can check dynamically to determine whether an option has been enabled and what level. The BooleanSwitch is the simpler of the two and is set to off by default. To turn it on, you edit your application's <xxx>.config file as follows:

            <add name="MyBooleanSwitch" value="1" />

The .config file can be your web.config file if your application is a Web service, for example. With this set, you now create the BooleanSwitch object. This can be stored in a static variable and be accessible by any code wishing to use the switch. This BooleanSwitch data member is part of a configuration object used throughout the system to retrieve any application configuration information.

public static BooleanSwitch MyBooleanSwitch = new
"This is my boolean switch");

The first parameter of the BooleanSwitch is the name of the switch. This must match the name used in your configuration setting. Setting this to 1 turns on the switch; conversely, setting this to zero turns it off. Once the switch is created, the BooleanSwitch can be used to help determined things such as tracing. Instead of calling Trace.Write or Trace.WriteLine, you now call Trace.WriteLineIf. The first parameter of this method is a Boolean that when set to true will cause tracing to occur. Instead of simply passing true or false directly, you use the BooleanSwitch, as follows:

Trace.WriteLineIf(MyBooleanSwitch.Enabled, "message to trace");

During execution, the code will use the BooleanSwitch to dynamically check the configuration setting. If the setting is set to 1, the BooleanSwitch returns true, and the trace occurs. No longer do you need to recompile your code.

A TraceSwitch works in the same fashion except you also get to control the level at which tracing should be enabled. To set a TraceSwitch in the configuration file, you must specify the name of the switch (same as before) and the level at which to enable tracing. A TraceSwitch has four properties: TraceError, TraceWarning, TraceInfo, and TraceVerbose. Its configuration-level settings are:

0 (off), 1 (error), 2 (warning), 3 (info), OR 4 (verbose)

For example, to create a TraceSwitch to control all system tracing, use the following:

public static TraceSwitch TraceLevel = new
TraceSwitch("TraceLevel", "Tracing Level");

To configure this TraceSwitch to verbose, you would use the following:

   <add name="TraceLevel" value="4" />

To trace a message (only if verbose has been set), you would use the following:

Trace.WriteLineIf(TraceLevel.TraceVerbose, "some message");

This line will send a trace message only if the verbose level has been set in the configuration file. In fact, using any other trace level in the WriteIf call would trace the message because setting the configuration to verbose will send all traces. Setting this to verbose causes the TraceSwitch trace level to return true for TraceInfo, TraceWarning, or TraceError. If the configuration setting was 3 (TraceInfo), then all trace levels except TraceVerbose would return true and, thus, the trace would be sent. In this case, TraceInfo, TraceWarning, and TraceError would return true but TraceVerbose would return false. I will discuss the use of tracing, custom trace listeners, and trace switches in the following section.

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