Passing Bitmap from C# to C++ Unmanaged code

Question

I am trying without success to write code in C# to pass a bitmap to an unmanaged c++ DLL and return a POINT structure.

I have done a lot of research on the internet, but have not found the "Gotcha" article or piece of code to help me resolve my issue.

The best I have been able to come up with so far is an unmanaged c++ DLL, wrapped with a managed c++ dll, called by my C# app. In testing that, I can pass simple types, such as an integer and return an integer with no problem. Now for the issue I am having, the bitmap.

I have tried passing an HBITMAP (using the GetHBitmap() function of my bitmap in c#), but I get errors during compile of "cannot convert parameter 1 from 'System::IntPtr' to 'HBITMAP'" and "Class1.FindImage(void *, void *) is inaccessible due to it's protection level".

Here is some of my code:

Main App:

class Program
{
    [StructLayout(LayoutKind.Sequential)]
    public struct POINT
    {
        public int X;
        public int Y;

        public POINT(int x, int y)
        {
            this.X = x;
            this.Y = y;
        }

        public static implicit operator System.Drawing.Point(POINT p)
        {
            return new System.Drawing.Point(p.X, p.Y);
        }

        public static implicit operator POINT(System.Drawing.Point p)
        {
            return new POINT(p.X, p.Y);
        }
    }

    static void Main(string[] args)
    {
        Image src = Image.FromFile("Pin.png");
        Image tgt = Image.FromFile("screen.png");

        Bitmap bsrc = new Bitmap(src);
        Bitmap btgt = new Bitmap(tgt);

        IntPtr bs = bsrc.GetHbitmap();
        IntPtr bt = btgt.GetHbitmap();

        POINT p = Class1.FindImage(bs, bt);
    }
}

ImportWrap.h:

namespace ImportWrap {

public ref class Class1
{
public:
    static POINT FindImage(IntPtr source, IntPtr target);
};
}

ImportWrap.cpp:

static POINT FindImage(IntPtr source, IntPtr target)
{
return ImgFuncs::MyImgFuncs::FindImage(source, target);
}

ImgFuncs.h

typedef long LONG;
typedef void *PVOID;
typedef PVOID HANDLE;
typedef HANDLE HBITMAP;

typedef struct tagPOINT {
  LONG x;
  LONG y;
} POINT, *PPOINT;

namespace ImgFuncs
{

    class MyImgFuncs
    {
    public:
        static __declspec(dllexport) POINT FindImage(HBITMAP source, HBITMAP target);
    };
}

ImgFuncs.cpp

namespace ImgFuncs
{
POINT MyImgFuncs::FindImage(HBITMAP source, HBITMAP target)
{
    POINT p;

    p.x = 1;
    p.y = 2;
    return p;
}
}

What am I doing wrong, or am I going completely off the map with my approach? I would GLADLY entertain any suggestions regarding the correct way to code what I am trying to do. I have some C++ code that is used to find an image within another image that works quite fast. Unfortunately, even using lockbits in c#, it's not fast enough. So I would like to make use of the c++ code for the image search.

I am sure I will run into further snags, but might be able to handle it if I can get past this stumbling block. As you can see, my C++ knowledge is limited.

Answer 1

You cannot use native struct types as a return value, C# can't handle them. Casting from IntPtr to HBITMAP requires a double cast. Like this:

static System::Drawing::Point FindImage(IntPtr source, IntPtr target)
{
    POINT retval = ImgFuncs::MyImgFuncs::FindImage((HBITMAP)(void*)source, (HBITMAP)(void*)target);
    return System::Drawing::Point(retval.X, retval.Y);
}

Add an assembly reference to System.Drawing. You might then also want to consider passing a Bitmap^ instead of an IntPtr to make it even easier to use.

Answer 2

You could probably skip the managed C++ wrapper entirely and just call the unmanaged DLL from C# via P/Invoke. Put something like this in your C# class

[DllImport("YourUnamangedDll.dll")]
private static extern POINT FindImage(IntPtr source, IntPtr target);

I haven't tested this, so you may need to tweak it as needed, but that's the general idea.