Pointer of a C# object for unmanaged interop

Question

I am currently writing a wrapper for the PhysFS library, and I stumbled across a bit of troubles regarding the marshalling of managed objects. Take for example the PHYSFS_enumerateFilesCallback method, which takes a function pointer and a user-defined pointer as its arguments. How can I pass managed objects to this method? This is what I am currently doing:

// This is the delegate signature
public delegate void EnumFilesCallback(IntPtr data, string origdir, string fname);

// This is the method signature
[DllImport(DLL_NAME, CallingConvention = CallingConvention.Cdecl)]
public static extern void PHYSFS_enumerateFilesCallback(string dir, EnumFilesCallback c, IntPtr d);

Finally, this is what I'm doing to pass an arbitrary object to the method:

// I use the unsafe keyword because the whole Interop class is declared so.
// This code was taken from https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.gchandle(VS.71).aspx
public static void EnumerateFilesCallback(string dir, EnumFilesCallback c, object data)
{
  unsafe
  {
    GCHandle objHandle = GCHandle.Alloc(data);
    Interop.PHYSFS_enumerateFilesCallback(dir, c, (IntPtr)objHandle);
    objHandle.Free();
  }
}

When I run this code:

static void Enum(IntPtr d, string origdir, string fname )
{
  System.Runtime.InteropServices.GCHandle handle = (System.Runtime.InteropServices.GCHandle)d;
  TestClass c = (TestClass)handle.Target;
  Console.WriteLine("{0} {1}", origdir, fname);
}

static void Main(string[] args)
{
  PhysFS.Init("");
  PhysFS.Mount("D:\\", "/hello", true);

  TestClass x = new TestClass() { a = 3, b = 4 }; // This can be any gibberish object

  PhysFS.EnumerateFilesCallback("/hello/", Enum, x);
}

The delegate gets called 4 times with legit data, the fifth time it contains garbage data and then it throws an AccessViolationException I suspect this is because the object gets GCed in between the calls to the delegate. Can anyone shed light on this?

UPDATE: Changing the mounted directory eliminates the rubbish data, yet the exception is still thrown, and still before all the data can be consumed

Looks like multiple problems, you'll definitely crash like this when the GC collects the delegate object that you asked the C# compiler to create for the `Enum` target. You have to store it. Smart thing to do here is to just use the existing C# wrapper, you'll find it in the *extras* subdirectory of the physfs source distribution. — Hans Passant, Jun 28 '15 at 18:01
@HansPassant I already checked that library out, and I decided to write my own because that one didn't satisfy me :D In fact, it is a bit lacklustre and misses, for example, this function., therefore it is not of much help in solving this problem... — Francesco Bertolaccini, Jun 28 '15 at 18:05
It is *very* hard to explain why it works at all, you should always get garbage arguments in the callback. Makes it hard to answer reliably of course. Well, good luck with it. — Hans Passant, Jun 28 '15 at 18:10
I'd like to point out that I took the code from [this MSDN article](https://msdn.microsoft.com/en-us/library/system.runtime.interopservices.gchandle(VS.71).aspx), so this method must, to an extent, be valid. I understand I'm on an edge case, though. — Francesco Bertolaccini, Jun 28 '15 at 18:13

score 0 · Answer 1 · answered Aug 06 '15 at 15:42

0

Have you tried to create the callback and store it as a class static field?

private static EnumFilesCallback callback = new EnumFilesCallback(Enum);

And in your main method:

PhysFS.EnumerateFilesCallback("/hello/", callback, x);

This should probably avoid the GC to collect the local variable holding the delegate object.

answered Aug 06 '15 at 15:42

thepirat000

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Thanks for the suggestion, I will try it as soon as I can, which unfortunately won't be before September :( – Francesco Bertolaccini Aug 06 '15 at 15:51

score 0 · Accepted Answer · answered Jan 26 '16 at 19:06

Thanks to everyone who invested their time trying to provide an answer! I've finally found the source of the problem and solved it!

The problem was... I am a bit ashamed of it... calling convention. All the PInvoked methods were declared as cdecl while I forgot to declare the delegates as such, so it created unbalanced stacks and mayhem and whatnot...

Pointer of a C# object for unmanaged interop

2 Answers2