Explain this C# code: byte* p = (byte*) (void*) Scan0;

Question

I found the code from the net in which i cant understand this line:-

byte* p = (byte*)(void*)Scan0;

There Scan0 is System.IntPtr. It is code of C#.Net. Plz Explain the above line.

The complete code is given below. this is code to convert a image in grayscale.

public static Image GrayScale(Bitmap b)
    {

        BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

        int stride = bmData.Stride;
        System.IntPtr Scan0 = bmData.Scan0;

        unsafe
        {
            byte* p = (byte*)(void*)Scan0;

            int nOffset = stride - b.Width * 3;

            byte red, green, blue;

            for (int y = 0; y < b.Height; ++y)
            {
                for (int x = 0; x < b.Width; ++x)
                {
                    blue = p[0];
                    green = p[1];
                    red = p[2];

                    p[0] = p[1] = p[2] = (byte)(.299 * red + .587 * green + .114 * blue);

                    p += 3;
                }
                p += nOffset;
            }
        }

        b.UnlockBits(bmData);

        return (Image)b;
    }

I understand all the code but only have the problem on this line.

byte* p = (byte*)(void*)Scan0;

Answer 1

First converts the IntPtr to a void pointer. Then to a byte pointer. This is unsafe code.

More on unsafe code: http://msdn.microsoft.com/en-us/library/aa288474%28VS.71%29.aspx

As Robert Harvey notes, a pointer is a memory location. In C/C++ arrays are closely tied to this concept. When it does the square brackets, its basically is adjusting the address.

Answer 2

It looks bizarre but then I don't know C# that well. It may be that there's some problem with casting a System.IntPtr directly to a byte*, but not with System.IntPtr to void* or void* to byte*.

The end result is probably the same as if you cast int* to char* in C: the ability to derefence p to get a single byte of the integer (assuming CHAR_BIT is 8 in the C implementation).

Answer 3

Check out this tutorial on unsafe code. It will explain what the line of code means, and how it can be used for other types besides byte:

http://msdn.microsoft.com/en-us/library/aa288474(VS.71).aspx

Essentially, unsafe code allows you to use c-style pointers.

Answer 4

Your question sounds as if you are asking what the code is doing, but based on some of your comments I think you are after why it is casting first to a void pointer.

You are likely confused here because there is no reason to be casting Scan0 first to a void*. The cast to byte* would work just as well.

Answer 5

Would it be possible that you need to cast to void to avoid the object from executing some self-cast code on it?

Answer 6

IntPtr has an explicit operator (void*) that allows casting to void*. If you try to cast directly from IntPtr to something else the compiler will barf because only the void* cast is defined for the IntPtr class. See also IntPtr::ToPointer( ).

Casting from void* to byte* is allowed by the compiler because at this point if you don't know what you're doing you're already in trouble.