C / C++ / MFC

// Get bitmaps for both icons
GetIconInfo(hFirstIcon, &FirstInfo);
GetIconInfo(hSecondIcon, &SecondInfo);
.
// Prepare mem DCs
dcFirstIcon.CreateCompatibleDC(NULL);
dcSecondMask.CreateCompatibleDC(NULL);
dcSecondIcon.CretaeCompatibleDC(NULL);
dcSecondMask.CreateCompatibleDC(NULL);
.
// Select the Icon bitmaps into the appropriate dcs
dcFirstIcon.SelectObject(FirstInfo.hbmColor);
dcFirstMask.SelectObject(FirstInfo.hbmMask);
dcSecondIcon.SelectObject(SecondInfo.hbmColor);
dcSecondMask.SelectObject(SecondInfo.hbmMask);
.
// transparent BitBlt of one image on another (result in dcFirstIcon)
dcFirstIcon.BitBlt(0, 0, IconSize, IconSize, &dcSecondIcon, 0, 0, SRCINVERT);
dcFirstIcon.BitBlt(0, 0, IconSize, IconSize, &dcSecondMask, 0, 0, SRCAND);
dcFirstIcon.BitBlt(0, 0, IconSize, IconSize, &dcSecondIcon, 0, 0, SRCINVERT);
.
// Prepare the megred mask
...
.
// Create the new icon based on the updated bitmap and mask
FirstInfo.fIcon = true;
hMergedIcon = CreateIconIndirect(&FirstInfo);