在 C# 互操作中传递结构指针导致 NULL

Question

I am building a managed DLL for use in unmanaged environment (C/C++ app - FreeRDP). Interop works fine in most cases, but in one particular I am not able to pass a pointer to struct. In the API I have a struct:

typedef struct _IWTSListenerCallback IWTSListenerCallback;
struct _IWTSListenerCallback
{
    UINT(*OnNewChannelConnection)(IWTSListenerCallback* pListenerCallback,
                              IWTSVirtualChannel* pChannel,
                              BYTE* Data,
                              BOOL* pbAccept,
                              IWTSVirtualChannelCallback** ppCallback);
};

As well as a function I am calling:

UINT(*CreateListener)(IWTSVirtualChannelManager* pChannelMgr,
                      const char* pszChannelName,
                      ULONG ulFlags,
                      IWTSListenerCallback* pListenerCallback);

Both translated to C#:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate uint ListenerCallbackNewConnectionDelegate(IntPtr listenerCallback, IntPtr channel, [MarshalAs(UnmanagedType.LPArray)] byte[] data, IntPtr accept, ref IntPtr channelCallback);

[StructLayout(LayoutKind.Sequential)]
public struct IWTSListenerCallback
{
    [MarshalAs(UnmanagedType.FunctionPtr)]
    public ListenerCallbackNewConnectionDelegate OnNewChannelConnection;
}

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate uint ChannelManagerCreateListenerDelegate(IntPtr channelManager, [MarshalAs(UnmanagedType.LPStr)] string channelName, ulong flags, IntPtr listenerCallback);

[MarshalAs(UnmanagedType.FunctionPtr)]
public ChannelManagerCreateListenerDelegate CreateListener;

And execution code:

var callback = new IWTSListenerCallback();
callback.OnNewChannelConnection = NewChannelConnection;
var pCallback = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(IWTSListenerCallback)));
Marshal.StructureToPtr(callback, pCallback, false);
var ret = channelManager.CreateListener(pChannelManager, "TestChannel", 0, pCallback);

And while pChannelManager (which is a pointer I obtain from unmanaged code calling my DLL) and the string are sent through without any problems, the pointer I create here (pCallback) is assigned successfuly in C#, but it results in a NULL in unmanaged code.

I assume the problem is either with how I defined the struct, or how I defined the function (although the function is being called successfuly in unmanaged code). I use the method to create the pointer in exact same way as in another part of the DLL, and it works perfectly fine there when passed to unmanaged function.

EDIT: By @jdweng suggestion:

[StructLayout(LayoutKind.Sequential)]
public struct TestCall
{
    public IntPtr channelManager;
    [MarshalAs(UnmanagedType.LPStr)]
    public string channelName;
    public ulong flags;
    public IntPtr listenerCallback;
}

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate uint ChannelManagerCreateListenerDelegate(IntPtr testStructure);

var test = new TestCall();
test.channelManager = pChannelManager;
test.channelName = "TestChannel";
test.flags = 0;
test.listenerCallback = pCallback;
var pTest = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(FreeRDPTypes.TestCall)));
Marshal.StructureToPtr(test, pTest, false);
var ret = channelManager.CreateListener(pTest);

Didn't work.

EDIT2: Workaround! Only if you have access to original unmanaged code. I rearranged the function arguments so the structure pointers are first, like this:

UINT(*CreateListener)(IWTSVirtualChannelManager* pChannelMgr,
                      IWTSListenerCallback* pListenerCallback,
                      const char* pszChannelName,
                      ULONG ulFlags);

And it works! Probably a problem with offset.

Answer 1

It was a matter of offset. C/C++ ULONG was typedef unsigned long which I wrongly assumed corresponded to C# ulong , but in fact the first one is 4 bytes in Visual, while the other is 8 bytes, which resulted in 4 bytes offset. Fixed by changing ulong to uint and adding [MarshalAs(UnmanagedType.U4)] for good measure. Final look of the function I was calling inside C#:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate uint ChannelManagerCreateListenerDelegate(IntPtr channelManager, [MarshalAs(UnmanagedType.LPStr)] string channelName, [MarshalAs(UnmanagedType.U4)] uint flags, IntPtr listenerCallback);