save file without save dialog - c

Question

I have 3 global variables:
OPENFILENAME ofn;
TCHAR FileName[1024];
TCHAR Title[1024];
In WM_CREATE, I fill the information for ofn:
static TCHAR filter[] = TEXT("Bitmap Files (*.BMP)\0*.bmp\0") \
TEXT("All Files (*.*)\0*.*\0\0");
ofn.lStructSize = sizeof(OPENFILENAME);
ofn.hwndOwner = hwnd;
ofn.hInstance = NULL;
ofn.lpstrFilter = filter;
ofn.lpstrCustomFilter = NULL;
ofn.nMaxCustFilter = 0;
ofn.nFilterIndex = 0;
ofn.lpstrFile = NULL;
ofn.nMaxFile = 1024;
ofn.lpstrFileTitle = NULL;
ofn.nMaxFileTitle = 1024;
ofn.lpstrInitialDir = NULL;
ofn.lpstrTitle = NULL;
ofn.Flags = 0;
ofn.nFileOffset = 0;
ofn.nFileExtension = 0;
ofn.lpstrDefExt = TEXT("bmp");
ofn.lCustData = 0;
ofn.lpfnHook = NULL;
ofn.lpTemplateName = NULL;
In WM_COMMAND with case ID_FILE_SAVE:, I have
case ID_FILE_SAVE:
if (!SaveAsBmpDialog(hwnd, FileName, Title))
return 0;
SaveAsBmp(FileName);
return 0;
where SaveAsBmpDialog is
static bool SaveAsBmpDialog(HWND hwnd, TCHAR *fileName, TCHAR *title)
{
ofn.hwndOwner = hwnd;
ofn.lpstrFile = fileName;
ofn.lpstrFileTitle = title;
ofn.Flags = NULL;
return GetSaveFileName(&ofn);
}
It works fine to save HDC to a Bitmap file (which can be open).
But in WM_CHAR with wParam == 'p', I have
SaveAsBmp(FileName);
return 0;
I use this function when SaveAsBmpDialog is called at once before, so FileName is a full-path already.
But I get a Bitmap file, which can not be open. (Even if I delete the file first, it creates the file, but still cannot be open). The file size is as before, so I think WriteFile in SaveAsBmp works.
I don't understand why it doesn't work without GetSaveFileName(&ofn) in WM_CHAR.
What I want is to save HDC to a exist file without save dialog.
ps: In SaveAsBmp, I have
HANDLE hFile = CreateFile(FileName, GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
DWORD dwBytesWritten;
//SetFilePointer(hFile, 0, NULL, FILE_BEGIN);
WriteFile(hFile, &fileHeader, sizeof(BITMAPFILEHEADER), &dwBytesWritten, NULL);
WriteFile(hFile, &info.bmiHeader, sizeof(BITMAPINFOHEADER) + info.bmiHeader.biClrUsed * sizeof(RGBQUAD), &dwBytesWritten, NULL);
WriteFile(hFile, pPixels, (int) info.bmiHeader.biSizeImage, &dwBytesWritten, NULL);
CloseHandle(hFile);