SystemEvents Class

Definition

public ref class SystemEvents sealed

public sealed class SystemEvents

type SystemEvents = class

Public NotInheritable Class SystemEvents

Inheritance

Object

SystemEvents

Examples

This section contains two examples. The first example shows how to use system events in an ordinary application, and the second example shows how to use system events in a Windows service.

Example 1

The following code example registers interest in some system events and then waits for any of those events to occur. The output shown occurs if the user changes the display resolution.

#using <System.dll>

using namespace System;
using namespace Microsoft::Win32;

// This method is called when a user preference changes.
void SystemEvents_UserPreferenceChanging(Object^ sender,
 UserPreferenceChangingEventArgs^ e)
 {
 Console::WriteLine("The user preference is changing. Category={0}",
 e->Category);
 }

// This method is called when the palette changes.
void SystemEvents_PaletteChanged(Object^ sender, EventArgs^ e)
{
 Console::WriteLine("The palette changed.");
}

// This method is called when the display settings change.
void SystemEvents_DisplaySettingsChanged(Object^ sender,
 EventArgs^ e)
{
 Console::WriteLine("The display settings changed.");
}

int main()
{
 // Set the SystemEvents class to receive event notification
 // when a user preference changes, the palette changes, or
 // when display settings change.
 SystemEvents::UserPreferenceChanging += gcnew
 UserPreferenceChangingEventHandler(
 SystemEvents_UserPreferenceChanging);
 SystemEvents::PaletteChanged += gcnew
 EventHandler(SystemEvents_PaletteChanged);
 SystemEvents::DisplaySettingsChanged += gcnew
 EventHandler(SystemEvents_DisplaySettingsChanged);

 // For demonstration purposes, this application sits idle
 // waiting for events.
 Console::WriteLine("This application is waiting for system events.");
 Console::WriteLine("Press <Enter> to terminate this application.");
 Console::ReadLine();
}

// This code produces the following output.
//
// This app is waiting for system events.
// Press <Enter> to terminate this application.
// Display Settings changed.
// User preference is changing. Category=General

using System;
using Microsoft.Win32;

public sealed class App 
{
 static void Main() 
 { 
 // Set the SystemEvents class to receive event notification when a user 
 // preference changes, the palette changes, or when display settings change.
 SystemEvents.UserPreferenceChanging += new 
 UserPreferenceChangingEventHandler(SystemEvents_UserPreferenceChanging);
 SystemEvents.PaletteChanged += new 
 EventHandler(SystemEvents_PaletteChanged);
 SystemEvents.DisplaySettingsChanged += new 
 EventHandler(SystemEvents_DisplaySettingsChanged); 

 // For demonstration purposes, this application sits idle waiting for events.
 Console.WriteLine("This application is waiting for system events.");
 Console.WriteLine("Press <Enter> to terminate this application.");
 Console.ReadLine();
 }

 // This method is called when a user preference changes.
 static void SystemEvents_UserPreferenceChanging(object sender, UserPreferenceChangingEventArgs e) 
 {
 Console.WriteLine("The user preference is changing. Category={0}", e.Category);
 }

 // This method is called when the palette changes.
 static void SystemEvents_PaletteChanged(object sender, EventArgs e)
 {
 Console.WriteLine("The palette changed.");
 }

 // This method is called when the display settings change.
 static void SystemEvents_DisplaySettingsChanged(object sender, EventArgs e)
 {
 Console.WriteLine("The display settings changed.");
 }
}

// This code produces the following output.
// 
// This app is waiting for system events.
// Press <Enter> to terminate this application.
// Display Settings changed.
// User preference is changing. Category=General

Imports Microsoft.Win32
Imports System.Windows.Forms

Friend Class Form1
 Inherits System.Windows.Forms.Form


 Public Sub New()
 MyBase.New()

 'This call is required by the Windows Form Designer.
 InitializeComponent()

 'Set the SystemEvents class to receive event notification 
 'when a user preference changes, the palette changes, or 
 'when display settings change.
 AddHandler SystemEvents.UserPreferenceChanging, _
 AddressOf SystemEvents_UserPreferenceChanging

 AddHandler SystemEvents.PaletteChanged, _
 AddressOf SystemEvents_PaletteChanged

 AddHandler SystemEvents.DisplaySettingsChanged, _
 AddressOf SystemEvents_DisplaySettingsChanged

 End Sub

 'Form overrides dispose to clean up the component list.
 Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
 If disposing Then
 If (components IsNot Nothing) Then
 components.Dispose()
 End If
 End If
 MyBase.Dispose(disposing)
 End Sub


 Private components As System.ComponentModel.IContainer

 <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()

 Me.SuspendLayout()

 '
 'Form1
 '
 Me.ClientSize = New System.Drawing.Size(648, 398)
 Me.Name = "Form1"
 Me.Text = "Form1"
 Me.ResumeLayout(False)

 End Sub


 ' This method is called when a user preference changes.
 Private Sub SystemEvents_UserPreferenceChanging( _
 ByVal sender As Object, _
 ByVal e As UserPreferenceChangingEventArgs)

 MessageBox.Show("UserPreferenceChanging: " & _
 e.Category.ToString())
 End Sub


 ' This method is called when the palette changes.
 Private Sub SystemEvents_PaletteChanged( _
 ByVal sender As Object, _
 ByVal e As EventArgs)

 MessageBox.Show("PaletteChanged")

 End Sub


 ' This method is called when the display settings change.
 Private Sub SystemEvents_DisplaySettingsChanged( _
 ByVal sender As Object, _
 ByVal e As EventArgs)

 MessageBox.Show("The display settings changed.")

 End Sub

End Class

Example 2

The following code example demonstrates a very simple Windows service that handles the TimeChanged and UserPreferenceChanged events. The example includes a service named SimpleService, a form named HiddenForm, and an installer. The form provides the message loop that is required by system events.

The service in this example starts a thread that runs an instance of HiddenForm. The events are hooked up and handled in the form. The events must be hooked up in the load event of the form, to make sure that the form is completely loaded first; otherwise the events will not be raised.

To run the example:

1. Compile the code from the command line. The name that you use for the source file is not important.

2. Install the service from the command line using the Installutil.exe (Installer Tool) utility. For example, InstallUtil example.exe if the source file name is example.cs or example.vb. You must be an administrator to install the service.

3. Use the Services console to start the service.

4. Change the system time, or change user preferences, such as mouse properties.

5. View the messages in the Application category of Event Viewer.

6. Use the Services console to stop the service.

7. Uninstall the service from the command line by using the /u option. For example, InstallUtil /u example.exe.

using System;
using System.ServiceProcess;
using System.Threading;
using System.Windows.Forms;
using System.Diagnostics;
using Microsoft.Win32;
using System.ComponentModel;
using System.Configuration.Install;

namespace SimpleServiceCs
{
 public class SimpleService : ServiceBase
 {
 static void Main(string[] args)
 {
 ServiceBase.Run(new SimpleService());
 }

 protected override void OnStart(string[] args)
 {
 EventLog.WriteEntry("SimpleService", "Starting SimpleService");
 new Thread(RunMessagePump).Start();
 }

 void RunMessagePump()
 {
 EventLog.WriteEntry("SimpleService.MessagePump", "Starting SimpleService Message Pump");
 Application.Run(new HiddenForm());
 }

 protected override void OnStop()
 {
 Application.Exit();
 }
 }

 public partial class HiddenForm : Form
 {
 public HiddenForm()
 {
 InitializeComponent();
 }

 private void HiddenForm_Load(object sender, EventArgs e)
 {
 SystemEvents.TimeChanged += new EventHandler(SystemEvents_TimeChanged);
 SystemEvents.UserPreferenceChanged += new UserPreferenceChangedEventHandler(SystemEvents_UPCChanged);
 }

 private void HiddenForm_FormClosing(object sender, FormClosingEventArgs e)
 {
 SystemEvents.TimeChanged -= new EventHandler(SystemEvents_TimeChanged);
 SystemEvents.UserPreferenceChanged -= new UserPreferenceChangedEventHandler(SystemEvents_UPCChanged);
 }

 private void SystemEvents_TimeChanged(object sender, EventArgs e)
 {
 EventLog.WriteEntry("SimpleService.TimeChanged", "Time changed; it is now " +
 DateTime.Now.ToLongTimeString());
 }

 private void SystemEvents_UPCChanged(object sender, UserPreferenceChangedEventArgs e)
 {
 EventLog.WriteEntry("SimpleService.UserPreferenceChanged", e.Category.ToString());
 }
 }

 partial class HiddenForm
 {
 private System.ComponentModel.IContainer components = null;

 protected override void Dispose(bool disposing)
 {
 if (disposing && (components != null))
 {
 components.Dispose();
 }
 base.Dispose(disposing);
 }

 private void InitializeComponent()
 {
 this.SuspendLayout();
 this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
 this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
 this.ClientSize = new System.Drawing.Size(0, 0);
 this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None;
 this.Name = "HiddenForm";
 this.Text = "HiddenForm";
 this.WindowState = System.Windows.Forms.FormWindowState.Minimized;
 this.Load += new System.EventHandler(this.HiddenForm_Load);
 this.FormClosing += new System.Windows.Forms.FormClosingEventHandler(this.HiddenForm_FormClosing);
 this.ResumeLayout(false);
 }
 }

 [RunInstaller(true)]
 public class SimpleInstaller : Installer
 {
 private ServiceInstaller serviceInstaller;
 private ServiceProcessInstaller processInstaller;

 public SimpleInstaller()
 {
 processInstaller = new ServiceProcessInstaller();
 serviceInstaller = new ServiceInstaller();

 // Service will run under system account
 processInstaller.Account = ServiceAccount.LocalSystem;

 // Service will have Start Type of Manual
 serviceInstaller.StartType = ServiceStartMode.Automatic;

 serviceInstaller.ServiceName = "Simple Service";

 Installers.Add(serviceInstaller);
 Installers.Add(processInstaller);
 }
 }
}

Imports System.ServiceProcess
Imports System.Threading
Imports System.Windows.Forms
Imports System.Diagnostics
Imports Microsoft.Win32
Imports System.ComponentModel
Imports System.Configuration.Install

Namespace SimpleServiceVb
 Public Class SimpleService
 Inherits ServiceBase
 
 Shared Sub Main(ByVal args() As String) 
 ServiceBase.Run(New SimpleService())
 End Sub
 
 Protected Overrides Sub OnStart(ByVal args() As String) 
 EventLog.WriteEntry("SimpleService", "Starting SimpleService")
 Dim t As New Thread(AddressOf RunMessagePump)
 t.Start() 
 End Sub
 
 Sub RunMessagePump() 
 EventLog.WriteEntry("SimpleService.MessagePump", _
 "Starting SimpleService Message Pump")
 Application.Run(New HiddenForm())
 End Sub
 
 Protected Overrides Sub OnStop() 
 Application.Exit()
 End Sub
 End Class 

 Partial Class HiddenForm
 Inherits Form

 Public Sub New() 
 InitializeComponent()
 End Sub
 
 Private Sub HiddenForm_Load(ByVal sender As Object, ByVal e As EventArgs) 
 AddHandler SystemEvents.TimeChanged, AddressOf SystemEvents_TimeChanged
 AddHandler SystemEvents.UserPreferenceChanged, AddressOf SystemEvents_UPCChanged
 End Sub 
 
 Private Sub HiddenForm_FormClosing(ByVal sender As Object, ByVal e As FormClosingEventArgs) 
 RemoveHandler SystemEvents.TimeChanged, New EventHandler(AddressOf SystemEvents_TimeChanged)
 RemoveHandler SystemEvents.UserPreferenceChanged, _
 New UserPreferenceChangedEventHandler(AddressOf SystemEvents_UPCChanged)
 End Sub 
 
 Private Sub SystemEvents_TimeChanged(ByVal sender As Object, ByVal e As EventArgs) 
 EventLog.WriteEntry("SimpleService.TimeChanged", _
 "Time changed; it is now " & DateTime.Now.ToLongTimeString())
 End Sub 
 
 Private Sub SystemEvents_UPCChanged(ByVal sender As Object, ByVal e As UserPreferenceChangedEventArgs) 
 EventLog.WriteEntry("SimpleService.UserPreferenceChanged", e.Category.ToString())
 End Sub 
 End Class

 Partial Class HiddenForm

 Private components As System.ComponentModel.IContainer = Nothing
 
 Protected Overrides Sub Dispose(ByVal disposing As Boolean) 
 If disposing AndAlso Not (components Is Nothing) Then
 components.Dispose()
 End If
 MyBase.Dispose(disposing)
 End Sub
 
 Private Sub InitializeComponent() 
 Me.SuspendLayout()
 Me.AutoScaleDimensions = New System.Drawing.SizeF(6F, 13F)
 Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
 Me.ClientSize = New System.Drawing.Size(0, 0)
 Me.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None
 Me.Name = "HiddenForm"
 Me.Text = "HiddenForm"
 Me.WindowState = System.Windows.Forms.FormWindowState.Minimized
 AddHandler Me.Load, AddressOf Me.HiddenForm_Load
 AddHandler Me.FormClosing, AddressOf Me.HiddenForm_FormClosing
 Me.ResumeLayout(False)
 End Sub 
 End Class 

 <RunInstaller(True)> _
 Public Class SimpleInstaller
 Inherits Installer

 Private serviceInstaller As ServiceInstaller
 Private processInstaller As ServiceProcessInstaller
 
 Public Sub New() 
 processInstaller = New ServiceProcessInstaller()
 serviceInstaller = New ServiceInstaller()
 
 ' Service will run under system account
 processInstaller.Account = ServiceAccount.LocalSystem
 
 ' Service will have Start Type of Manual
 serviceInstaller.StartType = ServiceStartMode.Automatic
 
 serviceInstaller.ServiceName = "Simple Service"
 
 Installers.Add(serviceInstaller)
 Installers.Add(processInstaller)
 End Sub
 End Class 
End Namespace

Remarks

The SystemEvents class provides the ability to respond to specific types of system events.

When a system event is raised, any delegates attached to the event are called using the thread that monitors for system events. Therefore, you should make any calls from your event handlers thread-safe. If you need to call a system event that is not exposed as a member of this class, you can use the InvokeOnEventsThread method.

See also

• PowerModeChangedEventHandler
• SessionEndedEventHandler
• SessionEndingEventHandler
• SessionSwitchEventHandler
• TimerElapsedEventHandler
• UserPreferenceChangedEventHandler
• UserPreferenceChangingEventHandler