Question

If I have a .Net Bitmap, I can create from it a GDI bitmap by calling the Bitmap‘s GetHbitmap() method.

Bitmap bmp = new Bitmap(100, 100);
IntPtr gdiBmp = bmp.GetHbitmap();

This works fine, but every time you call GetHbitmap, Windows has to allocate the new memory that the returned IntPtr references.

What I’d like to do – if possible – is write a function (I know PInvoke will be necessary here) that also generates a GDI bitmap copy of a Bitmap, but that overwrites an existing chunk of memory already referenced by an IntPtr returned from GetHbitmap instead of allocating new memory. So it would look something like this (if it were an extension method of Bitmap):

// desired method signature:
void OverwriteHbitmap(IntPtr gdi)
{

}

// ex:
Bitmap bmp1 = new Bitmap(100, 100);
IntPtr gdi1 = bmp1.GetHbitmap();

Bitmap bmp2 = new Bitmap(100, 100);
bmp2.OverwriteHbitmap(gdi1); // gdi1 is still pointing to the same block
    // of memory, which now contains the pixel data from bmp2

How can I do this? I assume I’ll need to know the structure of a GDI bitmap, and probably I can use LockBits and BitmapData for this, but I’m not sure exactly how.

Clues for the bounty hunters:

Bitmap has a method LockBits which locks the bitmap in memory and returns a BitmapData object. The BitmapData object has a Scan0 property which is an IntPtr pointing to the start of the locked bitmap’s pixel data (i.e it doesn’t point to the bitmap’s header a.k.a. the start of the bitmap itself).

I’m pretty sure the solution looks something like this:

Bitmap bmp1 = new Bitmap(100, 100);
IntPtr gdi1 = bmp1.GetHbitmap(); // now we have a pointer to a 
    // 100x100 GDI bitmap

Bitmap bmp2 = new Bitmap(100, 100);
BitmapData data = bmp2.LockBits();
IntPtr gdi1Data = gdi1 + 68; // magic number = whatever the size 
    // of a GDI bitmap header is
CopyMemory(data.Scan0, gdi1Data, 40000);

The solution does not have to be generic – it only needs to work for bitmaps with pixel format Format32bppArgb (the default GDI+ format).

Answer 1

Create a Graphics from the IntPtr with Graphics.FromHdc and use Graphics.DrawImage to paste the Bitmap into it.