Develop Reference

Marshal C# Method to Unity3D with C++

I would like to call a specific method in my Unity3D project within an external plugin. Here is my C++ code:
static void UNITY_INTERFACE_API OnRenderEventThreadSafe(int eventID)
{
[...] Some C++ code
//Call C# Method here
}
My C++ code does already work on its own but now I want to call a specific C# method. How can I manage to do this? I tried creating delegates like this
#if UNITY_EDITOR
[DllImport("lib")]
#else
[DllImport ("lib")]
#endif
private static extern void Method(Delegate callback);
private delegate void Delegate(int number);
[MonoPInvokeCallback(typeof(Delegate))]
private static void DelegateMessageReceived(int number)
{
Debug.Log("MessageReceived " + number);
}
public static void Method2()
{
Method(DelegateMessageReceived);
}
But I don't think this is the right way...
Should have mentioned it earlier, but I'm coding on Linux-Fedora25

I ran into the-same issue in the past when making rendering plugin but the solution is not really hard.
You need to create a global static function pointer to hold the C# function you want to call from C++. The C++ function pointer must be static. Initialize this variable from the Start or Awake function in C#. After this, you can freely call that function anytime from C++.
C++ (YourCppPlugin.h)
#define DLLExport __declspec(dllexport)
extern "C"
{
//Create a function pointer and give it a callback alias to shorten it
typedef void(*FuncCallBack)(int eventID);
//Create a callback delegate from the alias to hold the function from C#
static FuncCallBack callbackInstance = nullptr;
//Create the function that will be called from C# to store the function
DLLExport void RegisterRenderCallback(FuncCallBack cb);
}
C++ (YourCppPlugin.cpp):
#include "YourCPP.h"
//Save the function we received from C# to the global variable for future use:
void RegisterRenderCallback(FuncCallBack cb) {
callbackInstance = cb;
}
Done. You can now use the callbackInstance variable to call the C# function anytime from the Unity's UNITY_INTERFACE_API OnRenderEventThreadSafe C++ API function. Just make sure it's not null before calling it like below:
static void UNITY_INTERFACE_API OnRenderEventThreadSafe(int eventID)
{
[...] Some C++ code
//Call C# Method here
if (callbackInstance != nullptr)
callbackInstance(eventID);
}
Register a function to it first from C# with the RegisterRenderCallback function first before using the callbackInstance variable. See example in the Start function below.
C# side of the code:
// Use this for initialization
void Start()
{
//Register the OnRenderCallback function to our C++ plugin for a callback
RegisterRenderCallback(OnRenderCallback);
}
//Function that registers a local C# function to a delegate in C# for later use
[DllImport("YourCppPlugin", CallingConvention = CallingConvention.Cdecl)]
static extern void RegisterRenderCallback(renderCallback cb);
//int param callback delegate that links to the C# function we want to call from C++
delegate void renderCallback(int eventID);
//Function we want to call from C#. Registered in the Start function
[MonoPInvokeCallback(typeof(renderCallback))]
static void OnRenderCallback(int eventID)
{
Debug.Log("Called from C# with ID: " + eventID);
}
Finally, if you have more than one function to register, you have to use List, array or dictionary to hold them on both C# and C++ side then iterate and call each one when necessary.

C# app crashes on exit after using C++ function with delegates

My C# app needs to talk with a DLL written in C++. I don't have the code of this DLL, but I have the code for a demo app (also in C++) that uses the DLL and works.
Here's the interesting code from the C++ demo app. It has two functions, one of which accepts a callback set of 4 delegates.
typedef BOOL (CBAPI* OPENSCANNERSDK)(HWND hwnd, TCallBackSet* callbackSet, wchar_t* configPath);
typedef void (CBAPI* CLOSESCANNERSDK)();
typedef struct TCallBackSet
{
TOnScannerStatusEvent scannerStatusEvent;
TOnScannerNotifyEvent scannerNotifyEvent;
TOnRFIDStatusEvent rfidStatusEvent;
TOnRFIDNotifyEvent rfidNotifyEvent;
} TCallBackSet;
typedef void (cdecl* TOnScannerStatusEvent ) (int scannerStatus);
typedef void (cdecl* TOnScannerNotifyEvent) (int scannerNotify, int lParam);
typedef void (cdecl* TOnRFIDStatusEvent ) (int rfidStatus);
typedef void (cdecl* TOnRFIDNotifyEvent ) (int rfidnotify);
So, I have to call OpenScannerSDK, pass a callback set with pointers to some functions, do some stuff, and finally call CloseScannerSDK.
I have declared this in my C# app like so:
[DllImport(DllName, CallingConvention = CallingConvention.Cdecl, EntryPoint = "_OpenScannerSDK")]
extern public static bool OpenScannerSDK(IntPtr hwnd, TCallBackSet callbackSet,
[MarshalAs(UnmanagedType.LPWStr)]string configPath);
[DllImport(DllName, CallingConvention = CallingConvention.Cdecl, EntryPoint = "_CloseScannerSDK")]
extern public static void CloseScannerSDK();
[StructLayout(LayoutKind.Sequential)]
public class TCallBackSet
{
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate void TOnScannerStatusEvent(int scannerStatus);
[MarshalAs(UnmanagedType.FunctionPtr)]
public TOnScannerStatusEvent ScannerStatusEvent;
(I have removed the other 3 callbacks for brevity)
}
Finally I use the library like so:
var callback = new TCallBackSet() { ... set the delegates ... }
OpenScannerSDK(IntPtr.Zero, callback, ".");
... do other stuff...
CloseScannerSDK();
All this seems to work - both OpenScannerSDK and CloseScannerSDK and all the others I use between them work correctly. The problem is that as soon as the application tries to exit, I get an APPCRASH in KERNELBASE.dll. I don't see any relevant information in the crash report file. I have noticed that if I do not call OpenScannerSDK, but just some other functions of the DLL that are not related to delegates, the APPCRASH doesn't happen.
I also tried GC.KeepAlive for the delegates, no effect.

I had a similar issue porting other C DLL to C# some years ago.
Function pointers in C# are represented as delegates. Internally, delegates are class instances and they're collected by GC the same way other objects are collected.
Probably in your DLL there's some method that "stops" the API, making it stopping invoking the C++ function pointers. You must call this before you close the application.
Probably your app collects the delegate objects and when the C++ DLL tries to invoke from unmanaged code, finds an invalid object reference.
It's interesting to keep the references in C# of these delegates in private fields, to avoid them to be collected when the application is running. The sympton of this issue is that the application crashes intermitently.
Hope this helps.

To anyone interested, here is the final working solution.
First, CallBackSet is defined like so:
[StructLayout(LayoutKind.Sequential)]
public class CallBackSet
{
public IntPtr ScannerStatusEvent;
public IntPtr ScannerNotifyEvent;
public IntPtr RfidStatusEvent;
public IntPtr RfidNotifyEvent;
}
And then:
OnScannerStatusEvent onScannerStatus = (ScannerStatus status) => {...};
OnScannerNotifyEvent onScannerNotify = (ScannerNotify notify, short lparam) => {...};
OnRfidStatusEvent onRfidStatus = (RfidStatus status) => {...};
OnRfidNotifyEvent onRfidNotify = (RfidNotify notify) => {...};
GCHandle.Alloc(onScannerStatus);
GCHandle.Alloc(onScannerNotify);
GCHandle.Alloc(onRfidStatus);
GCHandle.Alloc(onRfidNotify);
callbackSet = new CallBackSet()
{
RfidNotifyEvent = Marshal.GetFunctionPointerForDelegate(onRfidNotify),
RfidStatusEvent = Marshal.GetFunctionPointerForDelegate(onRfidStatus),
ScannerNotifyEvent = Marshal.GetFunctionPointerForDelegate(onScannerNotify),
ScannerStatusEvent = Marshal.GetFunctionPointerForDelegate(onScannerStatus)
};
callbackPtr = Marshal.AllocHGlobal(Marshal.SizeOf(callbackSet));
Marshal.StructureToPtr(callbackSet, callbackPtr, false);
OpenScannerSDK(IntPtr.Zero, callbackPtr, ".");

Upon further testing I found that actually OpenScannerSDK always returns True, which made me believe it was working correctly, but it may not be the case. I did several tests, including passing a TCallBackSet struct that does not have any members and OpenScannerSDK still returned True.
So it seems the problem is with TCallBackSet and these delegates. Something's wrong there. I tried passing the CallBackSet w and w/o ref, no difference. Noticed int in C++ is not the same as int in C#, so changed functions signatures to use short instead of int. None of these made any difference.

LoadLibrary and define callback to C DLL (function pointer)

I have a code like this one in a DLL:
int (*callback)(int a, int b);
void mainfunc()
{
callback(1, 2);
callback(3, 4);
}
To access this DLL from a C program, I used to do this:
#include <windows.h>
int callback(int a, int b) {return a+b;}
int main()
{
HANDLE dll = LoadLibrary("test.dll");
*(void**)GetProcAddress(dll, "callback") = (void*)callback;
((void(*))GetProcAddress(dll, "mainfunc"))();
FreeLibrary(dll);
}
The DLL is still C code, but the main program have switched to C#. How to deal with function pointer? How would that code be in C#?

callback is a global variable in the dll, so you can't[1] set it from c#.
You shouldn't be setting it from C either. What you should have, if callback really is shared, is a function like SetCallback( MyCallbackType callback ) to set it.
But probably, you want to pass the function to mainfunc.
In either case, this is easy to do in C#:
// C#
public delegate Int32 Callback( Int32 a, Int32 b );
[DllImport("test.dll")]
static extern void mainfunc( Callback callback );
// Or if you're setting the callback globally,
[DllImport("test.dll")]
static extern void SetCallback(Callback callback);
[DllImport("test.dll")]
static extern void mainfunc();
The take home message is that DllImport is smart enough to convert a delegate with a reasonably close signature to a C function pointer.
[1] Ok, so you can...dll injection sometimes uses one of a number of hacks along those lines, but it's all kinds of the wrong way to do it in any other situation.

calling C++ functions containing callbacks in C#

hey all im trying to get my head around calling this c++ function in c#:
BOOL __stdcall CodecStart(int hRadio,void __stdcall (*CallbackFunc)(void *),void *CallbackTarget);
this is from a WinRadio api found here http://www.winradio.com/home/g305_sdk.htm.
i did find that other people asked about calling this specific function on the net and they had:
public delegate void CallbackFunc( IntPtr p);
[DllImport("WRG305API.dll")]
public static extern bool CodecStart(int hRadio, CallbackFunc func, IntPtr CallbackTarget);
but i cant figure out how to implement this further.
any thoughts or guidance as to how to call this?
many thanks

Here's a simple implementation that will put it all together.
class WinRadioWrapper
{
public delegate void CallbackFunc( IntPtr pData );
[DllImport( "WRG305API.dll" )]
public static extern bool CodecStart( int hRadio, CallbackFunc func, IntPtr CallbackTarget );
public bool CodecStartTest(int hRadio)
{
bool bStarted = CodecStart( hRadio, MyCallbackFunc, IntPtr.Zero );
return bStarted;
}
// Note: this method will be called from a different thread!
static void MyCallbackFunc( IntPtr pData )
{
// Sophisticated work goes here...
}
}
Note that because MyCallbackFunc will be executed on a different thread, I chose to make is static. This way you won't be tempted to access WinRadioWrapper data members.
For simplicity I passed an IntPtr.Zero parameter to the callback, but this can point to any data that you'd like to use in the callback.[Please ignore this paragraph] Look into Marshal.StructureToPtr if you'd like to pass data to the callback, but make sure to also pin the data that you're passing in order to make sure it's not garbage-collected (see GCHandle for more details).
EDIT:
With the interesting words by svick (thanks!), I realize I was mixing a copied object with a pinned one.
So, to sort things out:
Marshal.StructureToPtr should be used if you want to copy an existing data structure and then pass it to the callback function.
If, on the other hand, you'd like to use and existing data structure (e.g. for modifying its content), the you should use GCHandle in order to pin it in memory and prevent it from being garbage-collected.This, however, will add some maintenance overhead for the GCHandle.

A callback function is a code that is called by a dll (you're importing in this case) that performs some functions. You also need to learn how to work with delegates in c#. You can implement the code like this:
public void MyCallback(IntPtr p)
{
//do something
}
and then your dll call will be like this:
[DllImport("WRG305API.dll")]
public static extern bool CodecStart(int hRadio, func, IntPtr CallbackTarget);
If you need more guidance, post the C++ version of the code you want to convert and we can help you with the C# version.

All you need to do is to create a c# function that matches the signature of the delegate you declared. Create a delegate, hold on to a reference to this delegate so it doesn't get garbage collected, and call the dll import with the delegate as the callback.
so you would having something like this:
public void MyCallback(IntPtr P)
{
//do something
}
// somewhere else in your code
var cb = new CallbackFunc(MyCallback);
CodecStart(..., cb, ...);

C#. How to pass message from unsafe callback to managed code?

Is there a simple example of how to pass messages from unsafe callback to managed code?
I have a proprietary dll which receives some messages packed in structs and all is coming to a callback function.
The example of usage is as follows but it calls unsafe code too. I want to pass the messages into my application which is all managed code.
*P.S. I have no experience in interop or unsafe code. I used to develop in C++ 8 yrs ago but remember very little from that nightmarish times :)
P.P.S. The application is loaded as hell, the original devs claim it processes 2mil messages per sec.. I need a most efficient solution.*
static unsafe int OnCoreCallback(IntPtr pSys, IntPtr pMsg)
{
// Alias structure pointers to the pointers passed in.
CoreSystem* pCoreSys = (CoreSystem*)pSys;
CoreMessage* pCoreMsg = (CoreMessage*)pMsg;
// message handler function.
if (pCoreMsg->MessageType == Core.MSG_STATUS)
OnCoreStatus(pCoreSys, pCoreMsg);
// Continue running
return (int)Core.CALLBACKRETURN_CONTINUE;
}
Thank you.

You can use Marshal class to deal with interop code.
example:
C:
void someFunction(int msgId, void* funcCallback)
{
//do something
funcCallback(msgId); //assuming that function signature is "void func(int)"
}
C#
[DllImport("yourDllname.dll")]
static extern someFunction(int msgId, IntPtr funcCallbackPtr);
public delegate FunctionCallback(int msgId);
public FunctionCallback functionCallback;
public void SomeFunction(int msgId, out FunctionCallback functionCallback)
{
IntPtr callbackPtr;
someFunction(msgId, callbackPtr);
functionCallback = Marshal.DelegateToPointer(callbackPtr);
}
you can call as:
SomeFunction(0, (msgIdx) => Console.WriteLine("messageProcessed"));
I hope I did it right. I did not try to compile it :)