Find out number of icons in an icon resource using Win32 API

Question

I have an *.ico file that contains multiple icons in different sizes linked to my executable as a resource. I use this resource to set my application's icon with RegisterClassEx(), i.e.:

wcx.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_ICON1));

In addition to that, I'd also like to convert all the single icons in this resource to ARGB pixel arrays. This should be possible by using GetDIBits() on the bitmap returned by GetIconInfo().

However, there is one problem: I need to find out the number of icons in the HICON handle returned by LoadIcon() as well as their sizes. I do not seem to find an API that takes a HICON handle and tells me how many icons there are actually in there and what their sizes are.

Is this possible somehow or do I need to go the hard way and parse the *.ico resource myself?

Answer 1

The original icon handling functions are ancient. They were introduced in 16-bit Windows and designed for a system that defined only one icon size. Therefore, most of those functions are unaware of the possibility of more than one icon size. To get to the differently sized icons in a resource additional work is required.

Following the ICO file format, consisting of an icon directory and the actual icon images, an icon resource consists of two parts as well: The icon directory of type RT_GROUP_ICON and the individual icons (RT_ICON). The directory is represented by the following structures:

#pragma pack( push )
#pragma pack( 2 )
typedef struct 
{
    WORD            idReserved;   // Reserved (must be 0)
    WORD            idType;       // Resource type (1 for icons)
    WORD            idCount;      // How many images?
    GRPICONDIRENTRY idEntries[1]; // The entries for each image
} GRPICONDIR, *LPGRPICONDIR;
#pragma pack( pop )

and

#pragma pack( push )
#pragma pack( 2 )
typedef struct
{
    BYTE   bWidth;               // Width, in pixels, of the image
    BYTE   bHeight;              // Height, in pixels, of the image
    BYTE   bColorCount;          // Number of colors in image (0 if >=8bpp)
    BYTE   bReserved;            // Reserved
    WORD   wPlanes;              // Color Planes
    WORD   wBitCount;            // Bits per pixel
    DWORD  dwBytesInRes;         // how many bytes in this resource?
    WORD   nID;                  // the ID
} GRPICONDIRENTRY, *LPGRPICONDIRENTRY;
#pragma pack( pop )

The icon directory for an icon group ID can be retrieved using the following code:

typedef std::list<GRPICONDIRENTRY> IconDirectory;

IconDirectory GetIconDirectory( HMODULE hMod, WORD Id ) {
    HRSRC hRsrc = FindResourceW( hMod, MAKEINTRESOURCE( Id ), RT_GROUP_ICON );
    HGLOBAL hGlobal = LoadResource( hMod, hRsrc );
    GRPICONDIR* lpGrpIconDir = (GRPICONDIR*)LockResource( hGlobal );

    IconDirectory dir;
    for ( size_t i = 0; i < lpGrpIconDir->idCount; ++i ) {
        dir.push_back( lpGrpIconDir->idEntries[ i ] );
    }
    return dir;
}

With the information from the icon directory the individual icons can be constructed with this code:

HICON LoadSpecificIcon( HMODULE hMod, WORD Id ) {
    HRSRC hRsrc = FindResourceW( hMod, MAKEINTRESOURCE( Id ), RT_ICON );
    HGLOBAL hGlobal = LoadResource( hMod, hRsrc );
    BYTE* lpData = (BYTE*)LockResource( hGlobal );
    DWORD dwSize = SizeofResource( hMod, hRsrc );

    HICON hIcon = CreateIconFromResourceEx( lpData, dwSize, TRUE, 0x00030000,
                                            0, 0, LR_DEFAULTCOLOR );
    return hIcon;
}

Putting all the pieces together, the following loads explorer.exe as a resource file, retrieves the first icon group with ID 101 and prints the information from the icon directory for each entry. It then creates the individual icons and outputs the xHotspot and yHotspot data. For icons, the hotspot is located in the center:

void PrintIconDirEntry( const GRPICONDIRENTRY& DirEntry ) {
    _wprintf_p( L"ID: %04d; width=%02d; height=%02d; bpp=%02d\n",
                DirEntry.nID,
                DirEntry.bWidth, DirEntry.bHeight, DirEntry.wBitCount );
}

void PrintIconInfo( HICON hIcon ) {
    ICONINFO ii = { 0 };
    GetIconInfo( hIcon, &ii );
    _wprintf_p( L"xHotspot=%02d; yHotspot=%02d\n", ii.xHotspot, ii.yHotspot );
}

typedef std::list<GRPICONDIRENTRY>::const_iterator IconDirectoryCIt;

int _tmain(int argc, _TCHAR* argv[])
{
    HMODULE hMod = LoadLibraryExW( L"C:\\Windows\\system32\\explorer.exe",
                                   NULL, LOAD_LIBRARY_AS_IMAGE_RESOURCE );
    IconDirectory dir = GetIconDirectory( hMod, 101 );
    for ( IconDirectoryCIt it = dir.begin(); it != dir.end(); ++it ) {
        PrintIconDirEntry( *it );
        HICON hIcon = LoadSpecificIcon( hMod, it->nID );
        PrintIconInfo( hIcon );
        DestroyIcon( hIcon );
    }
    return 0;
}

To sum this up: Retrieving all icon sizes and color variations for an icon resources involves two steps:

1. Retrieve the icon directory with information for all icons in an icon group.
2. Iterate over the icon directory and construct the individual icons using CreateIconFromResourceEx.

References:

• MSDN documentation for the icon format: Icons
• The Old New Thing: The evolution of the ICO file format, part 1: Monochrome beginnings
• The Old New Thing: The evolution of the ICO file format, part 2: Now in color!
• The Old New Thing: The evolution of the ICO file format, part 3: Alpha-blended images
• The Old New Thing: The evolution of the ICO file format, part 4: PNG images
• The Old New Thing: The format of icon resources
• The Old New Thing: How do I override the default icon selection algorithm?