将带有参数的 c# 回调方法传递给 c++ dll 会导致 System.ExecutionEngineException

Posted 2023-02-22

【中文标题】将带有参数的 c# 回调方法传递给 c++ dll 会导致 System.ExecutionEngineException

【英文标题】：Passing c# callback method with parameter to c++ dll leads to System.ExecutionEngineException

【发布时间】：2020-05-24 11:48:11

【问题描述】：

我正在使用c++ dll 进行一些后台计算，我正试图让它向我的调用C# 代码报告进度。

为此，我注册了一个回调方法，它接受 StringBuilder 作为参数（在网上发现这是一种正确的做法）。

这是我的“c++”代码：

// --------------------------------------------
// ----------------- C++ CODE ----------------- 
// --------------------------------------------
// ----------------- dll api methods
// a custom class to contain some progress report stuff... basically, most important is
// that it contains the callback as ProgressCallback _callback;
CustomEventHandler*  _eventHandler = NULL;

// definition of the callback type
typedef void(__stdcall* ProgressCallback)(char* log);

// method to register the callback method
int __stdcall SetCallbackFunction(ProgressCallback callback) 
    // from https://***.com/a/41910450/2490877
    #pragma EXPORT_FUNCTION
    // I encapsulated the callback into a custom class
    _eventHandler = new CustomEventHandler();
    _eventHandler->setEventHandler(callback);
    // test all is ok => no problem at this stage, all works great, the
    // passed-in callback is called with correct message. 
    logToCallback("All is ok while testing the method. So far so good!!");
    return 0;


// the long and slow method (note that I might call it several times from c# during the
// one run
int __stdcall DoLooongStuff() 
    // from https://***.com/a/41910450/2490877
    #pragma EXPORT_FUNCTION
    // ------ this is a LOOOONG method that regualrly logs stuff via the callback,
    // here an example....
    char buf[1000];
    sprintf_s(buf, "This is a sample progress log with some formats :%i %i %g", 1, 2, 3.1415);
    logToCallback(buf);
    // --- the above works a few times without any problem
    return 0;


//--- this is a static method I use to send progress messages back
static void logToCallback(char* message) 
    if (_eventHandler) 
        _eventHandler->logToCallback(message);
    


// --------------- CustomEventHandlerClass
// ------- class declaration ------
class CustomEventHandler 
    public:
        void setEventHandler(ProgressCallback callback);
        void logToCallback(char* message);
    protected:
        ProgressCallback _callback;


// ----- class implementation
// set the callback method
void CustomEventHandler::setEventHandler(ProgressCallback callback) 
    _callback = callback;

void CustomEventHandler::logToCallback(char* message) 
    if (_callback) 
        _callback(message); // <========= this is where the debugger stops:
        // no more info than the annoying System.ExecutionEngineException...
        // I've tried to pass a constant message like "1234" but got the same issue...
        //_callback("1234");
        // if however I remove the call to the callback, I don't get errors 
        // (I know this doesn't mean I don't have any...)
    

现在对于调用 c# 代码，我使用以下代码：

// --------------------------------------------
// ----------------- C# CODE ------------------ 
// --------------------------------------------
// ----- the delegate type to be passed to the dll
public delegate bool CallbackFunction([MarshalAs(UnmanagedType.LPStr)] StringBuilder log);


// ----- prepare to load the dll's methods (I only show the SetCallback code here, other api methods
//       are declared and loaded the same way)
private delegate int _SetCallbackFunction_(CallbackFunction func);
private _SetCallbackFunction_ SetCallbackFunction_Dll;

public int SetCallbackFunction(CallbackFunction func) 
  return SetCallbackFunction_Dll(func);


// loading methods
private T GetDelegate<T>(string procName) where T : class 
  IntPtr fp = GetProcAddress(_dllHandle, procName);
  return fp != IntPtr.Zero ? Marshal.GetDelegateForFunctionPointer(fp, typeof(T)) as T : null;


async Task loadDllMethods() 
    // load the delegates => great, it works!
    SetCallbackFunction_Dll = GetDelegate<_SetCallbackFunction_>("SetCallbackFunction");
    // set callback in DLL, calls the delegate once successfully...
    SetCallbackFunction(cSharpCallback);
    await doTask();


async Task doTask() 
    // start the long dll method, that fires the callback to monitor progress
    while (someConditionIsMet) 
        DoLooongStuff(); // => calls the dll with registered callback!
    



// the actual callback
bool cSharpCallback(StringBuilder strBuilder) 
    // this is called a few times successfully with the expected message!
    Console.WriteLine(strBuilder.ToString());
    return true;

我搜索了不同的线程以找出错误。由于“buf”尺寸太小，我遇到了一个错误，所以我只是确保它足够大。我还测试了“&_callback”总是指向同一个地方（它是！）。

我没有搜索选项，任何帮助将不胜感激。请注意，我是 dll 集成、编组等方面的专家，我把参考资料放在我找到提示的地方！

【问题讨论】：

使用string 而不是StringBuilder

@DavidHeffernan 我试过了，在同一时间遇到了完全相同的问题......也许错误在其他地方，但如果是这样，不知道在哪里......

【参考方案1】：

只要在 C++ 中使用回调，就需要确保传递给 C++ 的委托是活动的。只要使用相应的 C++ 回调，您就有责任保持委托 C# 对象处于活动状态：

someField = new CallbackFunction(cSharpCallback);
SetCallbackFunction(someField);

更好的是，只需使用Scapix Language Bridge，它会完全自动生成 C++ 到 C# 的绑定（包括回调）。免责声明：我是Scapix Language Bridge的作者。

【讨论】：

是的 - 正如我上面的分析器所述！

【参考方案2】：

感谢this post，我找到了问题的答案：

为了使非托管函数指针保持活动状态（防止 GC），您需要在变量中保存委托的实例

所以修改后的代码只在C#中

// -------------- PREVIOUS CODE
async Task loadDllMethods() 
    // load the delegates => great, it works!
    SetCallbackFunction_Dll = GetDelegate<_SetCallbackFunction_>("SetCallbackFunction");
    // set callback in DLL, calls the delegate once successfully...
    SetCallbackFunction(cSharpCallback);
    await doTask();


// -------------- WORKING CODE!!!!
// add static reference....
static CallbackFunction _callbackInstance = new CallbackFunction(cSharpCallback); // <==== Added reference to prevent GC!!! 
async Task loadDllMethods() 
    // load the delegates => great, it works!
    SetCallbackFunction_Dll = GetDelegate<_SetCallbackFunction_>("SetCallbackFunction");
    // create callback!!!
    SetCallbackFunction(_callbackInstance); // <====== pass the instance here, not the method itself!!!
    await doTask();

注意：我也将StringBuilder 更改为string！