Sending a Message - Win32 Programmer's Reference

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Sending a Message

The SendMessage function is used to send a message directly to a window procedure. SendMessage calls a window procedure and waits for that procedure to process the message and return a result.

A message can be sent to any window in the system; all that is required is a window handle. Windows uses the handle to determine which window procedure should receive the message.

Before processing a message that may have been sent from another thread, a window procedure should first call the InSendMessage function. If this function returns TRUE, the window procedure should call ReplyMessage before any function that causes the thread to yield control, as shown in the following example.

case WM_USER + 5:

if (InSendMessage())

ReplyMessage(TRUE);

DialogBox(hInst, "MyDialogBox", hwndMain, (DLGPROC) MyDlgProc);

break;

A number of messages can be sent to controls in a dialog box. These control messages set the appearance, behavior, and content of controls or retrieve information about controls. For example, the CB_ADDSTRING message can add a string to a combo box, and the BM_SETCHECK message can set the check state of a check box or radio button.

Use the SendDlgItemMessage function to send a message to a control, specifying the identifier of the control and the handle of the dialog box window that contains the control. The following example, taken from a dialog box procedure, copies a string from a combo box's edit control into its list box. The example uses SendDlgItemMessage to send a CB_ADDSTRING message to the combo box.

HWND hwndCombo;

int cTxtLen;

PSTR pszMem;

switch (uMsg)

{

case WM_COMMAND:

switch (LOWORD(wParam))

{

case IDD_ADDCBITEM:

// Get the handle of the combo box and the

// length of the string in the edit control

// of the combo box.

hwndCombo = GetDlgItem(hwndDlg, IDD_COMBO);

cTxtLen = GetWindowTextLength(hwndCombo);

// Allocate memory for the string and copy

// the string into the memory.

pszMem = (PSTR) VirtualAlloc((LPVOID) NULL,

(DWORD) (cTxtLen + 1), MEM_COMMIT,

PAGE_READWRITE);

GetWindowText(hwndCombo, pszMem,

cTxtLen + 1);

// Add the string to the list box of the

// combo box and remove the string from the

// edit control of the combo box.

if (*pszMem != NULL)

{

SendDlgItemMessage(hwndDlg, IDD_COMBO,

CB_ADDSTRING, 0,

(DWORD) ((LPSTR) pszMem));

SetWindowText(hwndCombo, (LPSTR) NULL);

}

// Free the memory and return.

VirtualFree(pszMem, 0, MEM_RELEASE);

return TRUE;

// Process other dialog box commands.

}

// Process other dialog box messages.

}

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