How to get an IntPtr to a struct?

Question

I've got a method with the signature

public int Copy(Texture texture, Rect? srcrect, Rect? dstrect)

Rect is a struct, but I need to allow the caller to pass null (or IntPtr.Zero) to the method as well.

I want to then pass it off to a DLL with the signature

[DllImport("SDL2.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "SDL_RenderCopy")]
internal static extern int RenderCopy(IntPtr renderer, IntPtr texture, IntPtr srcrect, IntPtr dstrect);

I was hoping I could do something like the following:

return SDL.RenderCopy(_ptr, texture._ptr, srcrect.HasValue ? (IntPtr)srcrect.Value : IntPtr.Zero, dstrect.HasValue ? (IntPtr)dstrect.Value : IntPtr.Zero);

But I can't cast the struct like that. Is there some other way I can get an IntPtr out of it?

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There alternative is to create 4 overloads:

• ref Rect, ref Rect
• IntPtr, IntPtr
• ref Rect, IntPtr
• IntPtr, ref Rect

which could get even messier if I ever need to pass more than 2 struct pointers.

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I came up with a solution, but I have some questions about it:

public int Copy(Texture texture, Rect? srcrect=null, Rect? dstrect=null)
{
    return SDL.RenderCopy(_ptr, texture._ptr, srcrect.HasValue ? StructToPtr(srcrect) : IntPtr.Zero, dstrect.HasValue ? StructToPtr(dstrect) : IntPtr.Zero);
}

private static IntPtr StructToPtr(object obj)
{
    var ptr = Marshal.AllocHGlobal(Marshal.SizeOf(obj));
    Marshal.StructureToPtr(obj, ptr, false);
    return ptr;
}

Had I used ref Rect I wouldn't have had to allocate memory for the struct -- what does that do differently than this does?

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I did some experimentation. The ref Rect solution runs at about the same speed as converting a Rect to an IntPtr generating an IntPtr for a Rect, which leads me to suspect that C# is doing something very similar under the hood when you use refs. As soon as I make it a Rect? and add the conditional logic to the method it runs up to 50% slower... so the 4-overload route would probably be the fastest. However, we're talking 100-150ms for 100K iterations, which means the method itself is super cheap, which is probably why the conditionals have such a notable impact. As such, I'm sticking with my custom StructToPtr solution as it's the simplest solution.

Answer 1

You want to use Marshal.StructureToPtr.

You will also have to allocate and deallocate memory for the struct.

A good blog on the subject can be found at http://www.developerfusion.com/article/84519/mastering-structs-in-c/