Custom error trace listener: Create the class library
 You first create a class library, in which you implement your custom logic. Thus, you can easily reference this logic in your project and use it directly in your web application.
 To create a class library for the custom trace listener, perform the following:
  - Open your project in Visual Studio.
  - From the main menu, click File » Add » New Project.
  - ClickVisual C# » Windows » Class Library.
  - Name the project ExternalLoggingand click Add.
  - Delete the Class1.cs file from the ExternalLogging project.