Extending the Logging Application Block

Microsoft Enterprise Library 5.0

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The Logging Application Block is designed to suit a variety of applications and to provide the most commonly used logging functions. You can extend the application block through designated extension points. Typically, these are custom classes, written by you, that implement a particular interface or derive from an abstract class. Because these custom classes exist in your application space, you do not need to modify or rebuild the application block. Instead, you designate your extensions using configuration settings. Additionally, with extension points, you can adapt the application block to suit the needs of any particular application.

You can extend the capabilities of the block by adding custom formatters, trace listeners, and log filters.

Custom Provider or Extension

Interface or Base Class

Log Entry Formatter


Trace Listener


Log Filter



For detailed information about how to integrate custom providers with the Enterprise Library configuration system and configuration tools see Creating Custom Providers for Enterprise Library.

The following procedure describes the general approach to extend the Logging Application Block.

To extend the Logging Application Block

  1. Create a new custom class and add it to your project.
  2. Make sure that the class implements the required interfaces, constructors, and methods.
  3. Add the custom object to the configuration of the Logging Application Block using the Enterprise Library configuration tools:
    • Specify your custom class as the type name.
    • Specify any custom configuration properties by modifying the attributes of the object.

Creating a Custom Log Entry Formatter

You can create a new log entry formatter by implementing the interface ILogFormatter. Your custom class must carry the attribute ConfigurationElementType with the type CustomFormatterData as the attribute parameter.

C# Copy Code
public class MyFormatter : ILogFormatter
Visual Basic Copy Code
<ConfigurationElementType(GetType(CustomFormatterData))> _
Public Class MyFormatter
  Implements ILogFormatter

Your class must implement the Format method that accepts a LogEntry instance and returns the formatted data as a string.

Creating a Custom Trace Listener

In some cases, the trace listeners provided with the Logging Application Block will not satisfy your application's requirements, and you will need to create your own trace listeners.

You can create a new trace listener by extending the CustomTraceListener class. Your custom class must carry the attribute ConfigurationElementType with the type CustomFormatterData as the attribute parameter.

C# Copy Code
public class MyTraceListener : CustomTraceListener
Visual Basic Copy Code
<ConfigurationElementType(GetType(CustomTraceListenerData))> _
Public Class MyTraceListener
  Inherits CustomTraceListener

Your class must override the TraceData method to format the LogEntry object and write the information to the output destination.

The Logging Application Block trace listeners derive from the TraceListener class in the System.Diagnostic namespace. This means that applications that do not use the application block can still use the application block trace listeners. To allow your custom trace listener to execute correctly when it is used outside the Logging Application Block, you should do the following:

  • In the TraceData method, test to see if the data parameter is of type LogEntry. If it is not of type LogEntry, use the ToString method to format the object.
  • Override the Write and WriteLine methods.

Creating a Custom Log Filter

You can implement a custom Log Filter by implementing the ILogFilter interface or by extending the LogFilter base class. However, you must be aware of an issue that can prevent application code from resolving the configured name of the provider - although this is only an issue where you wish to query the collection of filters when checking if a log entry will be specifically blocked by this filter.

The ILogFilter interface defines a Name property that should return the name of the instance of the custom log filter from the configuration. However, there currently is no way to retrieve that name from within your custom log filter. Instead, you can pass a key\value pair in the NameValueCollection received by the constructor, and use this to set the Name property of the filter. When configuring your custom log filter, you will have to duplicate the name: once for the actual name of that instance of the custom log filter, and again in the named property collection that is passed to the constructor.

If you intend to modify or extend the LogEntry class, you may want to use the ReflectedPropertyToken class. This formatting token allows you to log custom properties on custom classes that either derive from the LogEntry class or modify it. This token has the syntax {property(CustomPropertyName)}, and it uses reflection to obtain the value of the specified property and log it to the chosen trace listener. For example, you might have a property named FileName that is the name of the log file. Using this token allows you to insert this name into a log entry.