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Message Structure

Implements a Windows message.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

'Declaration
<SecurityPermissionAttribute(SecurityAction.LinkDemand, Flags := SecurityPermissionFlag.UnmanagedCode)> _
Public Structure Message

The Message type exposes the following members.

  NameDescription
Public propertyHWndGets or sets the window handle of the message.
Public propertyLParamSpecifies the LParam field of the message.
Public propertyMsgGets or sets the ID number for the message.
Public propertyResultSpecifies the value that is returned to Windows in response to handling the message.
Public propertyWParamGets or sets the WParam field of the message.
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  NameDescription
Public methodStatic memberCreateCreates a new Message.
Public methodEqualsDetermines whether the specified object is equal to the current object. (Overrides ValueType.Equals(Object).)
Public methodGetHashCodeReturns the hash code for this instance. (Overrides ValueType.GetHashCode.)
Public methodGetLParamGets the LParam value and converts the value to an object.
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodToStringInfrastructure. Returns a String that represents the current Message. (Overrides ValueType.ToString.)
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  NameDescription
Public operatorStatic memberEqualityDetermines whether two instances of Message are equal.
Public operatorStatic memberInequalityDetermines whether two instances of Message are not equal.
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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

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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