Message Structure
Collapse the table of content
Expand the table of content

Message Structure


Implements a Windows message.

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

[SecurityPermissionAttribute(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
public struct Message


Gets or sets the window handle of the message.


Specifies the LParam field of the message.


Gets or sets the ID number for the message.


Specifies the value that is returned to Windows in response to handling the message.


Gets or sets the WParam field of the message.

System_CAPS_pubmethodSystem_CAPS_staticCreate(IntPtr, Int32, IntPtr, IntPtr)

Creates a new Message.


Determines whether the specified object is equal to the current object.(Overrides ValueType.Equals(Object).)


Returns the hash code for this instance.(Overrides ValueType.GetHashCode().)


Gets the LParam value and converts the value to an object.


Gets the Type of the current instance.(Inherited from Object.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a String that represents the current Message.(Overrides ValueType.ToString().)

System_CAPS_puboperatorSystem_CAPS_staticEquality(Message, Message)

Determines whether two instances of Message are equal.

System_CAPS_puboperatorSystem_CAPS_staticInequality(Message, Message)

Determines whether two instances of Message are not equal.

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.

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;

        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.DrawString("Application is active", this.Font, SystemBrushes.ActiveCaptionText, 20,20);
                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.

            base.WndProc(ref m);

.NET Framework
Available since 1.1

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

Return to top
© 2015 Microsoft