Message Structure
Updated: July 2010
Implements a Windows message.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
The Message structure wraps messages that Windows sends. You can use this structure to wrap a message and assign it to the window procedure to be dispatched. You can also use this structure to get information about a message the system sends to your application or controls. For more information about Windows messages, see Messages and Message Queues.
You cannot create the Message directly. Instead, use the Create method. For the sake of efficiency, the Message uses its pool of existing Messages instead of instantiating a new one, if possible. However, if a Message is not available in the pool, a new one is instantiated.
The following code example demonstrates overriding the WndProc method to handle operating system messages identified in the Message. The WM_ACTIVATEAPP operating system message is handled in this example to know when another application is becoming active. For information about the available Message.Msg, Message.LParam, and Message.WParam values, see the MSG Structure documentation. For information about the actual constant values, see Message Constants.
Imports System Imports System.Drawing Imports System.Windows.Forms Namespace csTempWindowsApplication1 Public Class Form1 Inherits System.Windows.Forms.Form ' Constant value was found in the "windows.h" header file. Private Const WM_ACTIVATEAPP As Integer = &H1C Private appActive As Boolean = True <STAThread()> _ Shared Sub Main() Application.Run(New Form1()) End Sub 'Main Public Sub New() MyBase.New() Me.Size = New System.Drawing.Size(300, 300) Me.Text = "Form1" Me.Font = New System.Drawing.Font("Microsoft Sans Serif", 18.0!, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, CType(0, Byte)) End Sub Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs) ' Paint a string in different styles depending on whether the ' application is active. If (appActive) Then e.Graphics.FillRectangle(SystemBrushes.ActiveCaption, 20, 20, 260, 50) e.Graphics.DrawString("Application is active", Me.Font, SystemBrushes.ActiveCaptionText, 20, 20) Else e.Graphics.FillRectangle(SystemBrushes.InactiveCaption, 20, 20, 260, 50) e.Graphics.DrawString("Application is Inactive", Me.Font, SystemBrushes.ActiveCaptionText, 20, 20) End If End Sub <System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name:="FullTrust")> _ Protected Overrides Sub WndProc(ByRef m As Message) ' Listen for operating system messages Select Case (m.Msg) ' The WM_ACTIVATEAPP message occurs when the application ' becomes the active application or becomes inactive. Case WM_ACTIVATEAPP ' The WParam value identifies what is occurring. appActive = (m.WParam.ToInt32() <> 0) ' Invalidate to get new text painted. Me.Invalidate() End Select MyBase.WndProc(m) End Sub End Class End Namespace
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.