Implement callback C# functions for C++ DLL

Question

I\'m writing a DLL wrapper for my C++ library, to be called from C#. This wrapper should also have callback functions called from the library and implemented in C#. These functi

Answer 1

As of Visual Studio 2013 at least, there is a safe way to pass callbacks from C# to C++ and have C++ store them and invoke them asynchronously later from unmanaged code. What you can do is create a managed C++/CX class (e.g., named "CallbackManager") to hold the callback delegate references in a map, keyed off an enum value for each. Then your unmanaged code can retrieve a managed delegate reference from the managed C++/CX CallbackManager class via the delegate's associated enum value. That way you don't have to store raw function pointers and so you don't have to worry about the delegate being moved or garbage-collected: it stays in the managed heap throughout its lifecycle.

On the C++ side in CallbacksManager.h:

#include 
#include 

using namespace Platform;

namespace CPPCallbacks
{
    // define callback IDs; this is what unmanaged C++ code will pass to the managed CallbacksManager class to retrieve a delegate instance
    public enum class CXCallbackType
    {
        cbtLogMessage,
        cbtGetValueForSetting
        // TODO: add additional enum values as you add more callbacks
    }

    // defines the delegate signatures for our callbacks; these are visible to the C# side as well
    public delegate void LogMessageDelegate(int level, String^ message);
    public delegate bool GetValueForSettingDelegate(String^ settingName, String^* settingValueOut);
    // TODO: define additional callbacks here as you need them

     // Singleton WinRT class to manage C# callbacks; since this class is marked 'public' it is consumable from C# as well
    public ref class CXCallbacksManager sealed
    {
    private:
        CXCallbacksManager() { }  // this is private to prevent incorrect instantiation

    public:
        // public methods and properties are all consumable by C# as well
        virtual ~CXCallbacksManager() { }

        static property CXCallbacksManager^ Instance
        {
            CXCallbacksManager^ get();
        }

        bool UnregisterCallback(CXCallbackType cbType);
        void UnregisterAllCallbacks();
        Delegate^ GetCallback(CXCallbackType cbType);

        // define callback registration methods
        RegisterLogMessageCallback(LogMessageDelegate^ cb) { RegisterCallback(CXCallbackType::cbtLogMessage, cb); }
        RegisterGetValueForSettingCallback(GetValueForSettingDelegate^ cb) { RegisterCallback(CXCallbackType::GetValueForSetting, cb); }
        // TODO: define additional callback registration methods as you add more callbacks

    private:
        void RegisterCallback(CXCallbackType cbType, Delegate^ rCallbackFunc);

        typedef unordered_map CALLBACK_MAP;
        typedef pair CBType_Delegate_Pair;

        // Note: IntelliSense errors shown for static data is a Visual Studio IntellSense bug; the code below builds fine
        // See http://social.msdn.microsoft.com/Forums/windowsapps/en-US/b5d43215-459a-41d6-a85e-99e3c30a162e/about-static-member-of-ref-class?forum=winappswithnativecode
        static mutex s_singletonMutex;
        static CXCallbacksManager^ s_rInstance;

        mutex m_callbackMapMutex;
        CALLBACK_MAP m_callbacksMap;   // key=CallbackType, value = C# delegate (function) pointer
    };
}  

In CallbacksManager.cpp we implement the managed C++/CX class accessed by both C# and our unmanaged C++ code:

#include 
#include "CXCallbacksManager.h"

using namespace Platform;

namespace CPPCallbacks
{
    // define static class data
    CXCallbacksManager^ CXCallbacksManager::s_rInstance;
    mutex CXCallbacksManager::s_singletonMutex;

    // Returns our singleton instance; this method is thread-safe
    CXCallbacksManager^ CXCallbacksManager::Instance::get()
    {
        s_singletonMutex.lock();

        if (s_rInstance == nullptr)
            s_rInstance = ref new CXCallbacksManager();  // this lives until the application terminates

        s_singletonMutex.unlock();
        return s_rInstance;
    }

    // Register a C# callback; this method is thread-safe
    void CXCallbacksManager::RegisterCallback(const CXCallbackType cbType, Delegate^ rCallbackFunc)
    {
        _ASSERTE(rCallbackFunc);

        m_callbackMapMutex.lock();
        m_callbacksMap.insert(CBType_Delegate_Pair(cbType, rCallbackFunc)); 
        m_callbackMapMutex.unlock();
    }

    // Unregister a C# callback; this method is thread-safe
    // Returns: true on success, false if no callback was registered for callbackType
    bool CXCallbacksManager::UnregisterCallback(const CXCallbackType cbType)
    {
        m_callbackMapMutex.lock();
        const bool bRemoved = (m_callbacksMap.erase(cbType) > 0);
        m_callbackMapMutex.unlock();

        return bRemoved;
    }

    // Unregister all callbacks; this method is thread-safe
    void CXCallbacksManager::UnregisterAllCallbacks()
    {
        // must lock the map before iterating across it
        // Also, we can't change the contents of the map as we iterate across it, so we have to build a vector of all callback types in the map first.
        vector allCallbacksList;
        m_callbackMapMutex.lock();

        for (CALLBACK_MAP::const_iterator it = m_callbacksMap.begin(); it != m_callbacksMap.end(); it++)
            allCallbacksList.push_back(it->first);

        for (unsigned int i = 0; i < allCallbacksList.size(); i++)
        {
            CALLBACK_MAP::const_iterator it = m_callbacksMap.find(allCallbacksList[i]);
            if (it != m_callbacksMap.end())     // sanity check; should always succeed
                UnregisterCallback(it->first);
        }
        m_callbackMapMutex.unlock();
    }

    // Retrieve a registered C# callback; returns NULL if no callback registered for type
    Delegate^ CXCallbacksManager::GetCallback(const CXCallbackType cbType)
    {
        Delegate^ rCallbackFunc = nullptr;
        m_callbackMapMutex.lock();

        CALLBACK_MAP::const_iterator it = m_callbacksMap.find(cbType);
        if (it != m_callbacksMap.end())
            rCallbackFunc = it->second;
        else
            _ASSERTE(false);    // should never happen! This means the caller either forgot to register a callback for this cbType or already unregistered the callback for this cbType.

        m_callbackMapMutex.unlock();
        return rCallbackFunc;
    }
}

The delegate instances remain stored in the managed heap by our CXCallbacksManager class, so now it's easy and safe to store callbacks on the C++ side for unmanaged code to invoke later asynchronously. Here is the C# side registering two callbacks:

using CPPCallbacks;

namespace SomeAppName
{
    internal static class Callbacks
    {
        // invoked during app startup to register callbacks for unmanaged C++ code to invoke asynchronously
        internal static void RegisterCallbacks()
        {
            CPPCallbacks.CXCallbacksManager.Instance.RegisterLogMessageCallback(new LogMessageDelegate(LogMessageDelegateImpl));
            CPPCallbacks.CXCallbacksManager.Instance.RegisterGetValueForSettingCallback(new GetValueForSettingDelegate(GetValueForSettingDelegateImpl));
            // TODO: register additional callbacks as you add them
        }

        //-----------------------------------------------------------------
        // Callback delegate implementation methods are below; these are invoked by C++
        // Although these example implementations are in a static class, you could also pass delegate instances created 
        // from inside a non-static class, which would maintain their state just like any other instance method (i.e., they have a 'this' object).
        //-----------------------------------------------------------------

        private static void LogMessageDelegateImpl(int level, string message)
        {
            // This next line is shown for example purposes, but at this point you can do whatever you want because 
            // you are running in a normal C# delegate context.
            Logger.WriteLine(level, message);
        }

        private static bool GetValueForSettingDelegateImpl(String settingName, out String settingValueOut)
        {
            // This next line is shown for example purposes, but at this point you can do whatever you want because 
            // you are running in a normal C# delegate context.
            return Utils.RetrieveEncryptedSetting(settingName, out settingValueOut);   
        }
    };
}

Lastly, here is how to invoke your registered C# callbacks from unmanaged C++ code:

#include 
#include    // for CStringW
#include "CXCallbacksManager.h"

using namespace CPPCallbacks;

// this is an unmanaged C++ function in the same project as our CXCallbacksManager class
void LogMessage(LogLevel level, const wchar_t *pMsg)
{
    _ASSERTE(msg);

    auto rCallback = static_cast(CXCallbacksManager::Instance->GetCallback(CXCallbackType::cbtLogMessage));
    _ASSERTE(rCallback);
    rCallback(level, ref new String(pMsg));   // invokes C# method
}

// this is an unmanaged C++ function in the same project as our CXCallbacksManager class
// Sets settingValue to the value retrieved from C# for pSettingName
// Returns: true if the value existed and was set, false otherwise
bool GetValueForSetting(const wchar_t *pSettingName, CStringW &settingValue)
{
    bool bRetCode = false;

    auto rCallback = static_cast(CXCallbacksManager::Instance->GetCallback(CXCallbackType::cbtGetValueForSetting));
    _ASSERTE(rCallback);
    if (rCallback)    // sanity check; should never be null
    {
        String^ settingValueOut;
        bRetCode = rCallback(ref new String(pSettingName), &settingValueOut);

        // store the retrieved setting value to our unmanaged C++ CStringW output parameter
        settingValue = settingValueOut->Data(); 
    }
    return bRetCode;
}

This all works because although you cannot store a managed delegate reference as a member variable inside an unmanaged class, you can still retrieve and invoke a managed delegate from unmanaged code, which is what the above two native C++ methods do.