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[.net] Managed type to IntPtr

Started by PaulCesar Dec 6, 2009 at 10:32 AM 4 replies 2.6k views

Original Post

Dec 6, 2009 at 10:32 AM

Good Morning GameDev, I am not getting much luck on IRC lately, so I figured I would try here, as gamedev is usually fairly responsive. I am using C# and .NET 4.0 (MSVS 2010) I recently ran into the need to pass a managed type THROUGH an unmanaged callback. (EnumThreadWindows in this case). Both the function and its callback work with an IntPtr, and both "endpoints" are in managed code. So basically I need an IntPtr to a managed class (struct actually, although I am fairly positive its the same internally). Since both endpoints are in managed code, I do not believe the intptr even really needs to be valid in an unmanaged context. As a temporary fix I am passing an int in intptr's place, and storing that in an ad-hoc nasty collection. This _IS_ a huge bottleneck and truthfully dispicable code. It needs to go. Hopefully one of you .NET geniuses can help with this one :) Thanks, Richard edit: to further clarify, I am using the following PINVOKE signatures for the enumwindow code


        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool EnumThreadWindows(uint dwThreadId, EnumThreadDelegate lpfn, IntPtr lParam);

        public delegate bool EnumThreadDelegate(IntPtr hWnd, IntPtr lParam);

Promethium

Dec 6, 2009 at 11:02 AM

I assume it's the lParam thats giving you problems, since HWND's is the Handle member of a control. lParam is usually an int (DWORD in unmanaged Win32) and you can create an IntPtr from an Int32 directly.

If you need something more complex you can take a look at Marshal.StructureToPtr. It should do what you need.

PaulCesar

Dec 6, 2009 at 11:12 AM

Yes, you are correct it is using the lparam that is giving me problems. In C++ I would always simply use a void* to a class or structure on the heap, and cast it however I needed.

Anyways, I did some research based on what you said, and I thought I came up with the solution, but somehow it is returning the wrong results. Am I doing this wrong?:

        public void PollThreadWindows(UInt32 process_id, UInt32 thread_id)        {            ProcessThreadMap ptm = new ProcessThreadMap();            ptm.process_id = process_id;            ptm.thread_id = thread_id;            Trace.WriteLine("this: "+ptm.process_id.ToString());            IntPtr umPtm = Marshal.AllocHGlobal(Marshal.SizeOf(ptm));            Win32.EnumThreadWindows(thread_id, new Win32.EnumThreadDelegate(EnumThreadCallback), umPtm);            Marshal.FreeHGlobal(umPtm);        }        public bool EnumThreadCallback(IntPtr hWnd, IntPtr lParam)        {            ProcessThreadMap ptm = (ProcessThreadMap)Marshal.PtrToStructure(lParam, typeof(ProcessThreadMap));            Trace.WriteLine("becomes "+ptm.process_id.ToString());            //process_map[ptm.process_id].threads[ptm.thread_id].windows.Add((uint)hWnd, "test window");            return true;        }

an example of the trace is:

this: 1616
becomes 2155104

PaulCesar

Dec 6, 2009 at 11:17 AM

In case it matters, ProcessThreadMap is defined as:

        public struct ProcessThreadMap        {            public UInt32 process_id;            public UInt32 thread_id;        }

Mike.Popoloski

Dec 6, 2009 at 11:19 AM

StructureToPtr actually moves the given managed object into unmanaged memory. If you just want a pointer to the managed object while it's still on the managed heap, you can use GCHandle to pin the object and get a pointer to the pinned data. Once you're done you'll have to unpin it so that the garbage collector can continue to deal with it.

PaulCesar

Dec 6, 2009 at 11:34 AM

Mike,

Thank you, that worked PERFECTLY. I would still love to know why the previous code did not work though, if nothing else for shits and giggles. But thank you both so much for your quick response and awsome direction. Proves once again the best coders are on gamedev :)

Richard

Edit:

Funniest thing. The example on MSDN was doing something very similar to what I have. Almost like they had this in mind!

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