Introduction

This topic focuses on two things:

What is the difference between SendMessage and PostMessage
How to use SendMessage, PostMessage with WM_KEYDOWN. The way to find LParam and RParam parameters

To understand what is the difference between SendMessage and PostMessage, please look at the below table.

What is the Difference between SendMessage, PostMessage

SendMessage	PostMessage
Sends the specified message to a window or windows. The `SendMessage` function calls the window procedure for the specified window and does not return until the window procedure has processed the message (from MSDN)	Places (posts) a message in the message queue associated with the thread that created the specified window and returns without waiting for the thread to process the message (from MSDN)
Sequentially	Not sequentially
synchronous	asynchronous

How to Use SendMessage, PostMessage?

//
// ::PostMessage (HWND, WM_KEYDOWN, WPARAM, LPARAM);
//

//
// ::SendMessage (HWND, WM_KEYDOWN, WPARAM, LPARAM);
//

Parameters?

HWND is a handle you want to seed message to this window
WPARAM, you can view here: The virtual-key code of the nonsystem key.
LPARAM, you can know through Notepad program and Microsoft spy++

How to Find LParam

Next, Click to menu Spy -> FindWindows, using cross circle on the red frame.

Drag this cross circle to Notepad on this edit area:

Click choose Messages properties same as the below picture. Then click ok.

Let's press RETURN key to Edit Area, you can easily see WM_KEYDOWN message on Microsoft Spy++ Dash Broad. Then, right click to that message and choose properties, you can easily know LPARAM and WPARAM value.

My LPARAM value found: 0x0001C001 and RPARAM value: 0x0000000D

::PostMessage(hHandle, WM_KEYDOWN, VK_RETURN, 0x001C0001);

::SendMessage(hHandle, WM_KEYDOWN, VK_RETURN, 0x001C0001);

Using the Source Code

Using FindWindowA and FindWindowEx

API for get window handle:

HWND hwnd = FindWindowA("Notepad", NULL );
HWND hWndChild = ::FindWindowEx(hwnd, NULL, L"Edit", NULL);

/* *******************************************************************************
 Developer: zidane (huuvi168@gmail.com)
 Last Modified: 2015-09-01
 * ******************************************************************************/
BOOL CReadInfomationGameDlg::OnInitDialog()
{
    CDialog::OnInitDialog();

    // Add "About..." menu item to system menu.

    // IDM_ABOUTBOX must be in the system command range.
    ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
    ASSERT(IDM_ABOUTBOX < 0xF000);

    CMenu* pSysMenu = GetSystemMenu(FALSE);
    if (pSysMenu != NULL)
    {
        CString strAboutMenu;
        strAboutMenu.LoadString(IDS_ABOUTBOX);
        if (!strAboutMenu.IsEmpty())
        {
            pSysMenu->AppendMenu(MF_SEPARATOR);
            pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
        }
    }

    // Set the icon for this dialog.  The framework does this automatically
    // when the application's main window is not a dialog
    SetIcon(m_hIcon, TRUE);   // Set big icon
    SetIcon(m_hIcon, FALSE);  // Set small icon
    
    // TODO: Add extra initialization here
    CString strCommand[] =  
    {

        L"learn-tech-tips",
        L"http://learn-tech-tips.blogspot.com",
        L"huuvi168@gmail.com",
        L"zidane"
        L"http://www.codeproject.com/Tips/1025019/Get-Server-Side-Data-time-with-Cplusplus",
    };

    int length = sizeof(strCommand) / sizeof(strCommand[0]);
    for (int i=0; i<length; i++)
        m_cbCommand.AddString(strCommand[i]);
 
 
    return TRUE;  // return TRUE  unless you set the focus to a control
}

HWND CReadInfomationGameDlg::GetHandleOfWindow(void)
{ 
    HWND hwnd = FindWindowA("Windows abc Class", NULL ); 

    if (!hwnd) 
    {
        MessageBox(_T("Window abc not found"), _T("Message"), MB_OK 
                                                    | MB_ICONINFORMATION);  
        return NULL;
    } 
 
    return hwnd;
}

void CReadInfomationGameDlg::OnBnClickedButtonAddmessage()
{
    // TODO: Add your control notification handler code here
    HWND hHandle = GetHandleOfWindow();

    CString strCBText;
    int nIndex = m_cbCommand.GetCurSel();
 
    if (nIndex == -1) // cannot found text in comboBox, post NULL
    {
        TCHAR strTemp[255];  
        memset(strTemp, 0x00, sizeof(strTemp));
        GetDlgItemText(IDC_COMBO_COMMAND, strTemp, sizeof(strTemp));
        PostChatMessage(hHandle, strTemp);
    }
    else // found text in comboBox
    {
        m_cbCommand.GetLBText(nIndex, strCBText); 
        PostChatMessage(hHandle, strCBText);
    }
}

void CReadInfomationGameDlg::PostChatMessage(HWND hHandle, LPCTSTR szChatMsg)
{
     ::PostMessage(hHandle, WM_KEYDOWN, VK_RETURN, 0x001C0001);
     ::PostMessage(hHandle, WM_KEYDOWN, VK_DOWN, 0x00500001);

    while (szChatMsg[0])
    {
        ::PostMessage(hHandle, WM_CHAR, LOBYTE(szChatMsg[0]),0);
        szChatMsg++;
    }

    ::PostMessage(hHandle, WM_KEYDOWN, VK_RETURN, 0x001C0001);
}

Points of Interest

I like coding, I like researching new technology.

My tech blog is http://learn-tech-tips.blogspot.com.

My blog sharing experiences about technology tips and trick include: language programming: C+, C#, ....
Design skills: photoshop, Office: Excel, Outlook and .... other things!

If you have any feedback about this topic, please leave your comment, we can discuss about it. Smile | :)

SendMessage and PostMessage?