CreateDIBSection and BITMAPINFO with variable-length inline array

[harborsiem]

MultiIconLib.zip

In some closed issues about BITMAPINFO I did not found any usable information how to use the actual generated BITMAPINFO for the bmiColors field. The bmiColors has just a size of 1 RGBQUAD. But for my implementation I need up to 256 RGBQUADs. So I developed a solution for this problem. First I build a new struct called BITMAPINFO256 (see attached file). And her is an example how to use with a BITMAPINFO256:

//mEncoder has defined a property Header with type BITMAPINFOHEADER and a property Colors with type RGBQUAD[]
//Both properties are set before with useful values.

HDC hDCScreen = PInvoke.GetDC(HWND.Null);

BITMAPINFO256 bitmapInfo = new BITMAPINFO256();
void* bits;
bitmapInfo.bmiHeader = mEncoder.Header;
int quadsSize = mEncoder.Colors.Length * sizeof(RGBQUAD);
void* bmiColorsPtr = &bitmapInfo.__bmiColors;
fixed (RGBQUAD* rgbQuadsLocal = mEncoder.Colors)
   //PInvoke.RtlMoveMemory(bmiColorsPtr, rgbQuadsLocal, (nint)quadsSize);
   Buffer.MemoryCopy(rgbQuadsLocal, bmiColorsPtr, (long)quadsSize, (long)quadsSize);
BITMAPINFO* bitmapInfoPtr = (BITMAPINFO*)&bitmapInfo;

HDC hDCScreenOUTBmp = PInvoke.CreateCompatibleDC(hDCScreen);
HBITMAP hBitmapOUTBmp = PInvoke.CreateDIBSection(hDCScreenOUTBmp, bitmapInfoPtr, DIB_USAGE.DIB_RGB_COLORS, &bits, HANDLE.Null, 0);

[harborsiem]

Here is an other unsafe implementation without using an additional struct BITMAPINFO256

HDC hDCScreen = PInvoke.GetDC(HWND.Null);

// Image

int quadsSize = mEncoder.Colors.Length * sizeof(RGBQUAD);
int biSize = quadsSize > 0 ? sizeof(BITMAPINFO) + sizeof(RGBQUAD) * (quadsSize - 1) : sizeof(BITMAPINFO);
byte* b = stackalloc byte[biSize];
BITMAPINFO* pbi = (BITMAPINFO*)b;
pbi->bmiHeader = mEncoder.Header;
pbi->bmiHeader.biHeight /= 2;
if (quadsSize > 0)
{
        RGBQUAD* bmiColorsPtr = &pbi->bmiColors._0;
        fixed (RGBQUAD* rgbQuadsLocal = mEncoder.Colors)
             //PInvoke.RtlMoveMemory(bmiColorsPtr, rgbQuadsLocal, (nint)quadsSize);
             Buffer.MemoryCopy(rgbQuadsLocal, bmiColorsPtr, (long)quadsSize, (long)quadsSize);
}

HDC hDCScreenOUTBmp = PInvoke.CreateCompatibleDC(hDCScreen);
void* bits;
HBITMAP hBitmapOUTBmp = PInvoke.CreateDIBSection(hDCScreenOUTBmp, pbi, DIB_USAGE.DIB_RGB_COLORS, &bits, HANDLE.Null, 0);

[AArnott]

Thank you for sharing. FYI #387 is related to this.

[harborsiem]

Here is the implementation without using an additional struct BITMAPINFO256 that uses CsWin32 version >= 0.3.85-beta

HDC hDCScreen = PInvoke.GetDC(HWND.Null);

// Image

//CsWin32 version >= 0.3.85-beta
int rgbQuadLength = mEncoder.Colors.Length;
byte* b = stackalloc byte[BITMAPINFO.SizeOf(rgbQuadLength)];
BITMAPINFO* pbi = (BITMAPINFO*)b;
pbi->bmiHeader = mEncoder.Header;
pbi->bmiHeader.biHeight /= 2;
if (rgbQuadLength > 0)
{
    mEncoder.Colors.CopyTo(pbi->bmiColors.AsSpan(rgbQuadLength));
}

HDC hDCScreenOUTBmp = PInvoke.CreateCompatibleDC(hDCScreen);
void* bits;
HBITMAP hBitmapOUTBmp = PInvoke.CreateDIBSection(hDCScreenOUTBmp, pbi, DIB_USAGE.DIB_RGB_COLORS, &bits, HANDLE.Null, 0);

[AArnott]

I'm delighted to hear the update that this is working for you. And thank you for sharing the update to your code so that others can benefit.
FWIW, folks should take care when using stackalloc with a variable length to avoid blowing the stack.