TraceLog in VB.NET

Tracing with .NET
The .NET Framework includes classes and data types that make it easy to log trace messages—the logging infrastructure is right there for you. Figure 1 lists the .NET Framework classes that you use for tracing. The classes are part of the System.Diagnostics namespace. The Trace class exposes numerous static (shared) methods. The Write methods let you log a specific message, while Assert lets you log a message if a specified condition is false. The Fail method is similar to an Assert; the condition is always false, so the Fail method always logs the specified message.
The Trace class outputs your messages to objects in its Listeners collection. The framework includes EventLogTraceListener, which writes to an event log; TextWriterTraceListener, which writes to a text file; and DefaultTraceListener, an instance of which is added by default to the Trace class's Listeners collection. The DefaultTraceListener class's Write and WriteLine methods emit the message to the attached debugger, which may or may not display the message.
You can implement your own trace listener. You might want to receive trace output from your application running on a remote machine behind a firewall. In that case, you can write a trace listener that sends trace messages to your server over HTTP requests. Of course, this would significantly degrade your application's performance, but only while tracing is enabled.
While the Write and WriteLine methods provide the basic functionality you need to log trace messages, WriteIf and WriteLineIf let you control whether a message is to be logged based on the application's trace level setting. There are three ways that you can control tracing. You can define the compilation constant TRACE and set it to True, telling the compiler to keep your tracing code (for example, calls to Trace.WriteLine) in the compiled binary. You can do this using the compiler command-line parameter /d:TRACE=True or by checking the Define TRACE constant box in the Visual Studio project properties dialog, as shown in Figure 2. Setting this constant to False causes the compiler to strip off all tracing code from the compiled binary.


Figure 2 Enabling Tracing in Visual Studio .NET

Figure 3 shows the Microsoft intermediate language (MSIL) output for a method that was compiled with TRACE=True. You'll notice there's a call to System.Diagnostics.Trace::WriteLineIf. Compare that to Figure 4, which is the MSIL output for the same method compiled with TRACE=False, which causes all calls to the Trace class methods to be removed by the compiler. The TRACE compilation constant is an effective way to make debug or troubleshooting versions of your programs with lots of tracing code; to create a more efficient production version, you just have to change a single constant. What's more, you don't have to enclose all tracing calls in #If statements as you would in Visual Basic 6.0.

BooleanSwitch
The second way you can control tracing during execution of an app without having to recompile is using the BooleanSwitch class. You do this by specifying whether tracing is on or off in a registry value or in an environment variable. BooleanSwitch's constructor accepts two parameters: the switch's name and its description.

Private Shared BITraceBooleanSwitch As New _ 
BooleanSwitch("BITraceControl", _ 
 "Batch Importer Trace Control")

The switch is off by default. To turn it on, either set a registry value or an environment variable. The registry value for this particular switch is:

HKLM/Software/SOFTWARE/Microsoft/COMPlus/Switches/BITraceControl

Note that the value name matches the switch's name that you specify when instantiating the switch object. You set this value to 0 or 1 to turn the switch off or on, respectively. You can also set the environment variable _switch _BITraceControl to 0 or 1.
This switch has no effect until you use it in your own If...Then statement or with Trace.WriteLineIf. For example:

Trace.WriteLineIf( _ 
  BITraceBooleanSwitch.Enabled, _ 
  "this means the switch is" &_   
  "turned on")

This line will log the message "this means the switch is turned on" only if the switch is turned on. Otherwise, it'll simply ignore it. The switch value is read once during execution (an optimization to avoid repetitive reading from the registry), so if you change the registry setting, you'll need to restart the application for the change to take effect.

TraceSwitch
In most cases you'll have various types of trace messages like error, warning, and information messages. Use TraceSwitch to control which message levels are logged. Like the BooleanSwitch, you specify TraceSwitch's name when you instantiate it and you set its value in the registry or an environment variable. The possible values for a TraceSwitch are 0 (off), 1 (errors), 2 (warnings), 3 (information), and 4 (verbose).
TraceSwitch has four properties: TraceError, TraceInfo, TraceWarning, and TraceVerbose. Each of these properties returns True if messages of that type are allowed to be logged. If you set the tracing level to 3 (information) then TraceError, TraceWarning, and TraceInfo will all return True, while Trace Verbose will return False. You can use TraceSwitch with Trace.WriteLineIf like this:

Trace.WriteLineIf(BITraceLevel.TraceInfo, _ 
    "This is my informational message")

BITraceLevel is an object that is instantiated from TraceSwitch. Figure 5 demonstrates using Trace, EventLogListener, and TraceSwitch to write messages to the event log. Messages will appear in the application log on the local computer with the message source set to BatchImporter.

TraceTrack: http://msdn.microsoft.com/msdnmag/issues/01/07/vbnet/