Message Sender

Environment: VC6 SP3


It may be desirable sometimes (for instance, for testing purposes) to send a message to a window of some application. Initially, MessageSender was created just for sending such messages, normally WM_USER + XXX or WM_APP + XXX or some system messages not containing pointers. Later, I added more features, such as context and time customization. A few messages, such as WM_SETTEXT and WM_SETFONT, can also be customized. Their list is not so long, though. I did it just for my favorite ones.

Settings and Operations

Window selection

Three ways are available:

  • By handle
  • By window class and title
  • Manual finding (MS Spy++-style window finding tool)

You can also navigate along next/previous or parent/first child windows, as well as back and forth through already found windows.

Message selection

Enter message code (hex) or select a message from the list and specify their wParam and lParam. For a few messages, you can customize their parameters in a more natural way. It is especially useful for those whose values should be pointers. (See, for instance, WM_SETTEXT or WM_SETFONT.)

Sending method

You can select among the SendMessage(), PostMessage(), and SendMessageTimeout() functions to send a message.


Some messages cannot be sent to windows from a different thread. If the Target window thread context is selected, MessageSender first installs a hook on the target window, injecting msgsndr.dll to its process address space. During the first call to the hook function, a new hidden window will be created in the target thread context. This window will serve as a medium while sending messages. Namely, MessageSender will pack an original message, send it packed by using WM_COPYDATA to that hidden window, and then the window procedure unpacks it and forwards it to its target. Note that the last steps will be performed not in MessageSender, but in the target thread context.

Time values

Normally, a message will be sent once, as soon as you press the “Send it!” button. By specifying corresponding values, you can do it later or a few times in a chain.

Using the Code

I hope that the code is more or less self-explanatory. The workspace contains two projects, MessageSender (exe) and msgsndr (injecting dll). The main application dialog is CMessageSenderDlg (files MessageSenderDlg.cpp and MessageSenderDlg.h).

It is possible that you would like to add customization for more messages. To protect my code from frequent changes, I derived CSendMessageDialog (files SendMessageDialog.h and SendMessageDialog.cpp) from CMessageSenderDlg. Implement accordingly some or all of its functions:

class CSendMessageDialog : public CMessageSenderDlg
    // TODO: add members keeping message specific values here

    // TODO: implement accordingly some or all next functions
    // to customize a message
    virtual void    AddConstantsToCombo();
                    // called from IniConstListCombo()
    virtual void    MessageChanged(UINT unMessage);
                    // called from OnChangeEditMessage()
    virtual void    CustomizeMessage(UINT unMessage);
                    // called from OnButtonCustomize()
    virtual void    PreSendMessage(SendingMessage*);
                    // called from OnButtonSend()
    virtual void    PrepareMessageLocal(SendingMessage*);
                    // called from SendMsgLocalContext()
    virtual CString DisplayMessageLocalResults(SendingMessage*);
                    // called from SendMsgLocalContext()
    virtual void    PrepareMessageTarget(SendingMessage*);
                    // called from SendMsgTargetContext()
    virtual CString DisplayMessageTargetResults(SendingMessage*);
                    // called from SendMsgTargetContext()

And, probably, also add necessary code to the InsertedDialogProc() in the DLL (file msgsndr\msgsndr.cpp)

I have marked the corresponding places by TODO comments.

If you do this job, please, do not forget to send me a copy!


Sending some messages may cause unpredictable results. Target applications may stop working properly, and you can lose some data. It is recommended to close all applications except those that you’re testing.


Download MessageSender – 42 Kb

Download source – 57 Kb

More by Author

Must Read