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RegisterWindowMessage function

Applies to: desktop apps only

Defines a new window message that is guaranteed to be unique throughout the system. The message value can be used when sending or posting messages.

Syntax

Copy 
UINT WINAPI RegisterWindowMessage(
  __in  LPCTSTR lpString
);

Parameters

lpString [in]

Type: LPCTSTR

The message to be registered.

Return value

Type:

Type: UINT

If the message is successfully registered, the return value is a message identifier in the range 0xC000 through 0xFFFF.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

The RegisterWindowMessage function is typically used to register messages for communicating between two cooperating applications.

If two different applications register the same message string, the applications return the same message value. The message remains registered until the session ends.

Only use RegisterWindowMessage when more than one application must process the same message. For sending private messages within a window class, an application can use any integer in the range WM_USER through 0x7FFF. (Messages in this range are private to a window class, not to an application. For example, predefined control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use values in this range.)

Examples

For an example, see Finding Text.

Requirements

Minimum supported client

Windows 2000 Professional

Minimum supported server

Windows 2000 Server

Header
Winuser.h (include Windows.h)

Library
User32.lib

DLL
User32.dll

Unicode and ANSI names

RegisterWindowMessageW (Unicode) and RegisterWindowMessageA (ANSI)

See also

Reference
PostMessage
SendMessage
Conceptual
Messages and Message Queues

 

 

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Build date: 2/3/2012

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C# Syntax 
The correct signature is:

[DllImport("user32.dll", CharSet = CharSet.Unicode)]
static extern uint RegisterWindowMessage(string lpProcName);

CharSet can be Unicode or Ansi and the above will work regardless of whether the target platform is 32 or 64 bits.
History
Return value is not an IntPtr
The x86 vs amd64 IntPtr advice above is wrong. (UINT) defined by Win32 is 32bits regardless of platform unlike (unsigned int) defined by the compiler that may vary with the target platform
History
Inverse of RegisterWindowMessage (GetClipboardFormatName)
If one wants to get the the message string (output) given the message identifier (input) returned by RegisterWindowMessage - i.e. a kind of inverse of RegisterWindowMessage - it is possible to use GetClipboardFormatName :

http://msdn2.microsoft.com/en-us/library/ms649040.aspx

In fact, doing a DUMPBIN /EXPORTS on user32.dll, it seems that RegisterWindowMessageA/W have the same RVA of RegisterClipboardFormatA/W.

So, basically, it seems that RegisterWindowMessage is just an "alias" for RegisterClipboardFormat...

For more details, read the thread on microsoft.public.vc.mfc newsgroup, with subject "Registered Window Message String", started on 2007, September 7th:

http://groups.google.it/group/microsoft.public.vc.mfc/browse_thread/thread/f83f7c12c80e4ada/460bc4c43a844a37

Giovanni

MSDN says: "The RegisterWindowMessage function defines a new window message that is guaranteed to be unique throughout the system."
On vista 64 RegisterWindowMessage(theSameString) function will return different values for programs run under localsystem and programs run under any other user.
::PostMessage(HWND_BROADCAST,registeredMessage) returns TRUE, but GetLastError gives access denied (error 5) when message is posted from an elevated process to a simple one.

Iohanson
History