I'm trying to create a Screen Capture DLL in C++ and send the resulting Byte Arrays to C#.
I'm able to get the size returned to C# but the byte array is always null.
Here's the C++ code (made up of bits i found on the internet)
__declspec(dllexport) int ScreenCap(BYTE* *data, DWORD *size)
{
try
{
//BITMAP bmpScreen;
HWND DesktopHwnd = GetDesktopWindow();
RECT DesktopParams;
HDC DevC = GetDC(DesktopHwnd);
GetWindowRect(DesktopHwnd,&DesktopParams);
DWORD Width = DesktopParams.right - DesktopParams.left;
DWORD Height = DesktopParams.bottom - DesktopParams.top;
DWORD FileSize = sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER)+(sizeof(RGBTRIPLE)+1*(Width*Height*4));
*size = FileSize;
char *BmpFileData = (char*)GlobalAlloc(0x0040,FileSize);
PBITMAPFILEHEADER BFileHeader = (PBITMAPFILEHEADER)BmpFileData;
PBITMAPINFOHEADER BInfoHeader = (PBITMAPINFOHEADER)&BmpFileData[sizeof(BITMAPFILEHEADER)];
BFileHeader->bfType = 0x4D42; // BM
BFileHeader->bfSize = sizeof(BITMAPFILEHEADER);
BFileHeader->bfOffBits = sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER);
BInfoHeader->biSize = sizeof(BITMAPINFOHEADER);
BInfoHeader->biPlanes = 1;
BInfoHeader->biBitCount = 24;
BInfoHeader->biCompression = BI_RGB;
BInfoHeader->biHeight = Height;
BInfoHeader->biWidth = Width;
RGBTRIPLE *Image = (RGBTRIPLE*)&BmpFileData[sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER)];
RGBTRIPLE color;
HDC CaptureDC = CreateCompatibleDC(DevC);
HBITMAP CaptureBitmap = CreateCompatibleBitmap(DevC,Width,Height);
SelectObject(CaptureDC,CaptureBitmap);
BOOL bRet = BitBlt(CaptureDC,0,0,Width,Height,DevC,0,0,SRCCOPY|CAPTUREBLT);
//GetDIBits(CaptureDC,CaptureBitmap,0,Height,Image,(LPBITMAPINFO)BInfoHeader, DIB_RGB_COLORS);
//GetObject(CaptureBitmap,sizeof(BITMAPFILEHEADER),&bmpScreen);
//BYTE* lpPixels = new BYTE[sizeof((LPBITMAPINFO)BInfoHeader)];
GetDIBits(CaptureDC, CaptureBitmap, 0, Height, *data, (LPBITMAPINFO)BInfoHeader, DIB_RGB_COLORS);
//DWORD Junk;
//DIBSECTION dib;
//GetObject(CaptureBitmap, sizeof(dib), (LPVOID)&dib);
//HANDLE FH = CreateFileA(BmpName,GENERIC_WRITE,FILE_SHARE_WRITE,0,CREATE_ALWAYS,0,0);
//WriteFile(FH,BmpFileData,FileSize,&Junk,0);
//CloseHandle(FH);
GlobalFree(BmpFileData);
return 1;
}
catch(char *p)
{
return 0;
}
}
And here's the C# code i'm using with the "ref" keyword.
[DllImport("consoleapplication1.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int ScreenCap(ref byte[] data, ref int size);
public static void CapScreen()
{
try
{
int hr = 0;
byte[] gData = null;
int gSize = 0;
hr = ScreenCap(ref gData, ref gSize);
int a = 1;
}
catch(Exception ex)
{
int a = 1;
}
I'm thinking there may be an issue with my GetDIBits but i'm not sure. I hope some of you guru's can set me on the right path. Thanks.
* EDIT *
OK Big thanks to both jdweng and filip for pointing me in the right direction. i'm now receiving the screen as an IntPtr and able to render it to my C# app. here's the code that now works (kind of i'll explain at the bottom)
__declspec(dllexport) char* ScreenCap(DWORD * size)
{ try { // Get screen dimensions int nScreenWidth = GetSystemMetrics(SM_CXSCREEN); int nScreenHeight = GetSystemMetrics(SM_CYSCREEN);
*size = ((((24 * nScreenWidth + 31)&(~31)) / 8)*nScreenHeight);
BITMAPINFO MyBMInfo = {0};
MyBMInfo.bmiHeader.biSize = sizeof(MyBMInfo.bmiHeader);
// Create compatible DC, create a compatible bitmap and copy the screen using BitBlt()
HDC hdcScreen = GetDC(GetDesktopWindow());
HDC hdcCompatible = CreateCompatibleDC(hdcScreen);
HBITMAP hBmp = CreateCompatibleBitmap(hdcScreen, nScreenWidth, nScreenHeight);
//HGDIOBJ hOldBmp = SelectObject(hdcCompatible, hBmp);
HGDIOBJ hOldBmp = (HGDIOBJ) SelectObject(hdcCompatible, hBmp);
BOOL bOK = BitBlt(hdcCompatible,0,0,nScreenWidth, nScreenHeight, hdcScreen,0,0,SRCCOPY|CAPTUREBLT);
SelectObject(hdcCompatible, hOldBmp); // always select the previously selected object once done
// Get the BITMAPINFO structure from the bitmap
GetDIBits(hdcScreen, hBmp, 0, 0, NULL, &MyBMInfo, DIB_RGB_COLORS);
// create the bitmap buffer
char* lpPixels = new char[MyBMInfo.bmiHeader.biSizeImage];
MyBMInfo.bmiHeader.biCompression = BI_RGB;
MyBMInfo.bmiHeader.biBitCount = 24;
// get the actual bitmap buffer
GetDIBits(hdcScreen, hBmp, 0, MyBMInfo.bmiHeader.biHeight, (LPVOID)lpPixels, &MyBMInfo, DIB_RGB_COLORS);
//Clean Up
DeleteDC(hdcCompatible);
ReleaseDC(GetDesktopWindow(), hdcScreen);
DeleteDC(hdcScreen);
DeleteObject(hBmp);
//DeleteObject(hOldBmp);
return lpPixels;
}
catch(char *p)
{
return 0;
}
}
and the C#
[DllImport("consoleapplication1.dll", CallingConvention = CallingConvention.Cdecl)]
static extern IntPtr ScreenCap(ref int size);
while(true)
{
int size = 0;
IntPtr ptr = ScreenCap(ref size);
byte[] result = new byte[size];
Marshal.Copy(ptr, result, 0, size);
Marshal.Release(ptr);
ptr = IntPtr.Zero;
MainWindow.Dispatcher.Invoke(new Action(
() =>
{
System.Windows.Media.ImageSource ThumbnailImage = System.Windows.Media.Imaging.BitmapSource.Create(1920, 1080, 96, 96, System.Windows.Media.PixelFormats.Bgr24, null, result, 1920 * 3);
MainWindow.canvasMain.Background = new System.Windows.Media.ImageBrush(ThumbnailImage);
result = null;
GC.Collect();
GC.WaitForPendingFinalizers();
}
), null);
}
Not sure if the problem i now face is due to doing it this way but i'm getting a major memory leak from the C++ dll now. If this is another issue all together, do i need to create a new thread?
** Edit **
Problem solved by creating a new function in the C++ dll to delete the returned char array. may not be elegant but it works.
i'd like to give a big thanks to Filip who's comments helped me to look in all sorts of places and see other things. Jdweng, your IntPtr was a god send. Not sure how i set this as answered by Jdweng. would love to award to you both.
