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Applications can use the EventLog.EntryWritten event to be notified when entries are written to the event log of the local computer; notifications from remote computers are not supported. First an EventLog instance is created and mapped to a specific log. Second the EnableRaisingEvents property is set to true; this enables event notifications on the EventLog instance. Finally the EventLog.EntryWritten event is provided with an event handler delegate to be notified when event log entries are written. The delegate that defines the EntryWritten event handler has the following signature:
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public delegate void EntryWrittenEventHandler(object sender, EntryWrittenEventArgs e);
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The sender argument is the EventLog instance that raised the event, and e is a System.Diagnostics.EntryWrittenEventArgs instance; EntryWrittenEventArgs contains an Entry property that gets an EventLogEntry representing the event log entry that caused the event to be raised. The following example program listens to the Application event log and displays summary details to the console each time an entry is written:
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using System; using System.Diagnostics; public class EventLoglListener { public static void Main() { // Create the EventLog instance and open the // "Application" log EventLog log = new EventLog("Application", ".", "testapp");
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Appendix A. Platform Integration // Enable event notifications log.EnableRaisingEvents = true; // Create the event handler log.EntryWritten += new EntryWrittenEventHandler(EventLogEventHandler); // Make sure the process does not terminate Console.ReadLine(); } public static void EventLogEventHandler(object sender, EntryWrittenEventArgs arg) { // Get the EventLogEntry that triggered the event EventLogEntry entry = arg.Entry; // Display summary information for the event to // the console Console.WriteLine( "{0}:Category {1} event triggered by {2}", entry.TimeWritten, entry.CategoryNumber, entry.Source);
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If we run the previous EventLogWriter example while the EventLogListener example is running, we get output similar to the following:
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02/04/2002 02/04/2002 02/04/2002 02/04/2002 15:04:06:Category 15:04:11:Category 15:04:16:Category 15:04:21:Category 0 0 0 0 event event event event triggered triggered triggered triggered by by by by MyApp MyApp MyApp MyApp
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A Windows Service is an application that runs as a background process independent of the currently logged on user. Services are supported on Windows NT, Windows 2000, and Windows XP. The life cycle and runtime configuration of a service is controlled by a component of Windows called the Service Control Manager (SCM) based on configuration information held in the Windows registry. The SCM also controls how other programs can interact with the service. Service development and deployment are straightforward but lack the elegant simplicity of many aspects of .NET. The complexity stems from the necessity to install the service instead of simply running it like a normal assembly. The creation of a service involves the development of the following three components: the service application, an installer, and a service controller. We discuss each component individually in the following sections.
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Appendix A. Platform Integration
The Service Application
Development of the service application involves the following steps: 1. Derive a class from System.ServiceProcess.ServiceBase that will contain the functionality for the service. 2. Implement an instance constructor that configures the service and sets the inherited properties of ServiceBase to the correct initial values. 3. Override the methods inherited from ServiceBase that process life cycle messages from the SCM; all services should override the OnStart method. 4. Implement a Main method that calls the static ServiceBase.Run method, passing an instance of the service class as an argument. Despite having a Main method, service code cannot be executed directly; the ServiceBase.Run call causes a Windows Service Start Failure dialog to appear. The service must be installed before it can execute; we cover installation in the next section. No lengthy processing should be carried out in the Main method or the instance constructor, nor should the service functionality be started until instructed by the SCM. The creation of a service is best explained by example, so we'll develop two simple services that exist in a single assembly. Despite being defined in a single assembly, each service is independently controlled via the SCM. We have called one service LogBeacon1 and the other LogBeacon2. Both the LogBeacon1 and LogBeacon2 services are instances of the BeaconService class that derives from ServiceBase. When instantiated, the BeaconService class creates a thread that periodically writes entries to the Application event log; the activity of the logging thread is controlled by commands from the SCM. The BeaconService code is listed here; a discussion of the code follows.
using System; using System.Threading; using System.ServiceProcess; public class BeaconService : ServiceBase { private Thread BeaconThread = null; private int Interval = 5000; public BeaconService(String name) { ServiceName = name; CanStop = true; CanPauseAndContinue = true; BeaconThread = new Thread(new ThreadStart(Beacon)); } protected override void OnStart(String[] args) { if (BeaconThread != null) { BeaconThread.Start(); } }
Appendix A. Platform Integration protected override void OnStop() { if (BeaconThread != null) { BeaconThread.Abort(); } } protected override void OnPause() { if (BeaconThread != null) { BeaconThread.Suspend(); } } protected override void OnContinue() { if (BeaconThread != null) { BeaconThread.Resume(); } } private void Beacon() { while (true) { EventLog.WriteEntry(ServiceName + " active at " + DateTime.Now); Thread.Sleep(Interval); } } public static void Main() { // Create an array to hold instances of each // Windows Service ServiceBase [] ServiceList = new ServiceBase [2]; ServiceList[0] = new BeaconService("LogBeacon1"); ServiceList[1] = new BeaconService("LogBeacon2"); ServiceBase.Run(ServiceList); } }
The Main method creates two BeaconService instances, one for each of the services. We pass the service name as an argument to the BeaconService constructor; this is used to set the value of the inherited ServiceName property. ServiceName is the name the SCM uses internally to identify the service. In the BeaconService constructor, we have set the value of the inherited CanStop and CanPauseAndContinue properties to true. The SCM uses these properties to determine the life cycle control messages that the service can process. These properties as well as the CanShutdown and CanHandlePowerEvent properties are summarized in Table A-10.
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