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Message Structure

Implements a Windows message.

Namespace: System.Windows.Forms
Assembly: System.Windows.Forms (in system.windows.forms.dll)

public struct Message
public final class Message extends ValueType
Not applicable.

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.

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. See the Platform SDK documentation reference located in the MSDN 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 Platform SDK (Core SDK section) download, which is also available on MSDN.

using System;
using System.Drawing;
using System.Windows.Forms;

namespace csTempWindowsApplication1
{
    public class Form1 : System.Windows.Forms.Form
    {
        // Constant value was found in the "windows.h" header file.
        private const int WM_ACTIVATEAPP = 0x001C;
        private bool appActive = true;

        [STAThread]
        static void Main() 
        {
            Application.Run(new Form1());
        }
        
        public Form1()
        {
            this.Size = new System.Drawing.Size(300,300);
            this.Text = "Form1";
            this.Font = new System.Drawing.Font("Microsoft Sans Serif", 18F, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, ((System.Byte)(0)));
        }

        protected override void OnPaint(PaintEventArgs e) 
        {
            // Paint a string in different styles depending on whether the
            // application is active.
            if (appActive) 
            {
                e.Graphics.FillRectangle(SystemBrushes.ActiveCaption,20,20,260,50);
                e.Graphics.DrawString("Application is active", this.Font, SystemBrushes.ActiveCaptionText, 20,20);
            }
            else 
            {
                e.Graphics.FillRectangle(SystemBrushes.InactiveCaption,20,20,260,50);
                e.Graphics.DrawString("Application is Inactive", this.Font, SystemBrushes.ActiveCaptionText, 20,20);
            }
        }

	[System.Security.Permissions.PermissionSet(System.Security.Permissions.SecurityAction.Demand, Name="FullTrust")]
        protected override void WndProc(ref Message m) 
        {
            // Listen for operating system messages.
            switch (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 = (((int)m.WParam != 0));

                    // Invalidate to get new text painted.
                    this.Invalidate();

                    break;                
            }
            base.WndProc(ref m);
        }
    }
}

package JSLTempWindowsApplication1; 

import System.*;
import System.Drawing.*;
import System.Windows.Forms.*;
import System.Security.Permissions.*;

public class Form1 extends System.Windows.Forms.Form
{
    // Constant value was found in the "windows.h" header file.
    private final int WM_ACTIVATEAPP = 0x1C;
    private boolean appActive = true;

    /** @attribute STAThread()
     */
    public static void main(String[] args)
    {
        Application.Run(new Form1());
    } //main

    public Form1()
    {
        this.set_Size(new System.Drawing.Size(300, 300));
        this.set_Text("Form1");
        this.set_Font(new System.Drawing.Font("Microsoft Sans Serif", 18, 
            System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, 
            (ubyte)0));
    } //Form1

    protected void OnPaint(PaintEventArgs e)
    {
        // Paint a string in different styles depending on whether the
        // application is active.
        if (appActive) {
            e.get_Graphics().FillRectangle(SystemBrushes.get_ActiveCaption(), 
                20, 20, 260, 50);
            e.get_Graphics().DrawString("Application is active", 
                this.get_Font(), SystemBrushes.get_ActiveCaptionText(), 
                20, 20);
        }
        else {
            e.get_Graphics().FillRectangle(SystemBrushes.get_InactiveCaption(), 
                20, 20, 260, 50);
            e.get_Graphics().DrawString("Application is Inactive", 
                this.get_Font(), SystemBrushes.get_ActiveCaptionText(),
                20, 20);
        }
    } //OnPaint

    /** @attribute SecurityPermissionAttribute(SecurityAction.Demand, Flags=SecurityPermissionFlag.UnmanagedCode)
    */
    protected void WndProc(/** @ref */Message m)
    {
        // Listen for operating system messages.
        switch (m.get_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.get_WParam().ToInt32() != 0;
                // Invalidate to get new text painted.
                this.Invalidate();
                break;
        }
        super.WndProc(m);
    } //WndProc
} //Form1

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

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

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