Marshalling array of structures that contain a std::vector

Question

I am trying to marshal a vector of the following C++ class to C# that is used in a tree:

class CFileNode
{
  private:
    std::string m_name;
    std::vector<CFileNode*> m_children;
    bool m_isDirectory;
    CFileNode* m_parent;
};

And:

extern DATAACCESSLAYERDLL_API size_t __stdcall Get_FileSystemNodeCount()
{
    // GetFileNodeList() returns a std::vector<CFileNode*>&
    return CFileSystem::GetFileNodeList().size();
}

extern DATAACCESSLAYERDLL_API void __stdcall Get_FileSystemNodes(size_t count, CFileNode** nodes)
{
    nodes = CFileSystem::GetFileNodeList().data();
}

And in C#:

[SuppressUnmanagedCodeSecurity]
[DllImport("MyDll.dll")]
static private extern int Get_FileSystemNodeCount();

[SuppressUnmanagedCodeSecurity]
[DllImport("MyDll.dll")]
static private extern void Get_FileSystemNodes(int count, [In,Out] IntPtr[] outNodes);

And my struct:

[StructLayout(LayoutKind.Sequential, Pack = 1), Serializable]
public struct CFileNode
{
    [MarshalAs(UnmanagedType.LPStr)]
    string m_name;
    [MarshalAs(UnmanagedType.SafeArray)]
    IntPtr[] m_children;
    [MarshalAs(UnmanagedType.Bool)]
    bool m_isDirectory;
    [MarshalAs(UnmanagedType.LPStruct)]
    IntPtr m_parent;
}

And where I use it:

int count = Get_FileSystemNodeCount();
IntPtr[] results = new IntPtr[count];
Get_FileSystemNodes(count, results);

foreach (IntPtr ptr in results)
{
    m_fileSystemNodes.Add(Marshal.PtrToStructure<CFileNode>(ptr));
}

In its current state, the IntPtr[] results is just a big array of zeroes. If I modify C++ GetFileSystemNodes() to go through the vector and add everything to the array nodes, results will have a load of numbers that look like memory addresses (although I have no idea if they are garbage or not), but then the Marshal.PtrToStructure<CFileNode>(ptr) fails with an AccessViolationException.

I would like to transfer this whole tree structure from C++ to C# in as few PInvoke calls as possible, for speed. This is why, before any of this code is executed, my entire tree is flattened out into a list. Each node still has a pointer to its parent and children, so it's easy to reconstruct it. If it makes it easier, I could also just pass the root node.

Here are my questions:

1) Is marshalling vectors of objects that contain other vectors even possible? Should I make my C++ classes simpler, using arrays or something instead?

2) Is there a way to pass the vector of nodes through GetFileSystemNodes without moving everything in the vector to the array argument?

3) What is the correct way to write my C# struct so it matches my C++ one?

Related/duplicate?: stackoverflow.com/questions/4747171/… - accepted answer suggests to use e.g.: C++/CLI instead (which would have been my first instinct as well) — UnholySheep
– UnholySheep, Commented Dec 16, 2016 at 15:16
And the official documentation also suggests to prefer C++/CLI over PInvoke — UnholySheep
– UnholySheep, Commented Dec 16, 2016 at 15:18

Tim B · Accepted Answer · 2016-12-16 15:12:57Z

1

This can be done, I would recommend using SWIG. SWIG makes interop between C++ and other languages (including C#) much easier. It has built in support to handle some of the C++ STL containers, such as vectors.

Yes, SWIG can handle vectors of objects, you just have to build them up in the interface file.

answered Dec 16, 2016 at 15:12

Tim B

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