WndProc Method
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Control.WndProc Method

Processes Windows messages.

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

<SecurityPermissionAttribute(SecurityAction.InheritanceDemand, Flags := SecurityPermissionFlag.UnmanagedCode)> _
<SecurityPermissionAttribute(SecurityAction.LinkDemand, Flags := SecurityPermissionFlag.UnmanagedCode)> _
Protected Overridable Sub WndProc ( _
	ByRef m As Message _
Dim m As Message 



Type: System.Windows.Forms.Message%

The Windows Message to process.

All messages are sent to the WndProc method after getting filtered through the PreProcessMessage method.

The WndProc method corresponds exactly to the Windows WindowProc function. For more information about processing Windows messages, see the WindowProc function.

Notes to Inheritors:

Inheriting controls should call the base class's WndProc method to process any messages that they do not handle.

The following code example demonstrates overriding the WndProc method to handle operating system messages identified in the Message structure. The WM_ACTIVATEAPP operating system message is handled in this example to know when another application is becoming active. Refer to the MSDN library at http://msdn.microsoft.com/library to understand the available Message.Msg, Message.LParam, and Message.WParam values. Actual constant values can be found in the Windows.h header file included in the Windows SDK, which is available in the MSDN library at http://msdn.microsoft.com/library.

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()

            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)
                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. 

            End Select 
        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.

.NET Framework

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

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