C++/CLI managed class member callback by managed code

Question

I am designing a class in C++/CLR that uses a callback function provided by user code.

This works very nicely if the callback function is free ( i.e. not the member of a class ). It is almost the same as in pure C++.

Here is some sample code that works well:

ref class ClassThatUsesCallback
{
public:
    typedef void (*callback_t)( String^ );
    void setCallback( callback_t pfun )
    {
        myCallback = pfun;
    }
    void Run()
    {
        if( myCallback != nullptr ) {
            myCallback("This is a test");
        }
    }
private:
    callback_t myCallback;
};

void FreeFunction( String^ s )
{
    Console::WriteLine( "Free Function Callback " + s );
}

int main(array<System::String ^> ^args)
{
   ClassThatUsesCallback^ theClassThatUsesCallback
       = gcnew ClassThatUsesCallback();
    theClassThatUsesCallback->setCallback( FreeFunction );
    theClassThatUsesCallback->Run();
}

However, I would like the callbacked function to be a member of a class in the user code ( so it can make use of and change the attributes of the user code class )

The following code does not compile

ref class ClassThatProvidesCallback
{
public:
    void MemberFunction( String^ s )
    {
        Console::WriteLine( "Member Function Callback " + s );
    }
    void Run()
    {
        ClassThatUsesCallback^ theClassThatUsesCallback
            = gcnew ClassThatUsesCallback();
        theClassThatUsesCallback->setCallback( 
            &ClassThatProvidesCallback::MemberFunction );
        theClassThatUsesCallback->Run();
    }

};

I get this error

error C3374: can't take address of 'ClassThatProvidesCallback::MemberFunction'
unless creating delegate instance

When I research this, I find numerous explanations of how to call un-managed code from managed code ( and vice-versa ) I do not need to do this - all the code involved is managed. So I am hoping that someone can point me to a simple way to this.

Of course, it is better to use standard events, which have the same functionality as callback and allow to have several subscribers. See EventHandler<TEventArgs> delegate. — Alex F
– Alex F, Commented Mar 10, 2014 at 15:58

Alex F · Accepted Answer · 2014-03-10 15:38:17Z

This is full solution:

ref class ClassThatUsesCallback
{
public:
    void setCallback( Action<String^>^ callback )
    {
        myCallback = callback;
    }
    void Run()
    {
        if( myCallback != nullptr ) {
            myCallback("This is a test");
        }
    }
private:
    Action<String^>^ myCallback;
};

ref class ClassThatProvidesCallback
{
public:
    void MemberFunction( String^ s )
    {
        Console::WriteLine( "Member Function Callback " + s );
    }
    void Run()
    {
        ClassThatUsesCallback^ theClassThatUsesCallback
            = gcnew ClassThatUsesCallback();
        theClassThatUsesCallback->setCallback(gcnew Action<String^>(this,
            &ClassThatProvidesCallback::MemberFunction));
        theClassThatUsesCallback->Run();
    }

};

int main(array<System::String ^> ^args)
{
    ClassThatProvidesCallback^ c = gcnew ClassThatProvidesCallback();
    c->Run();

    return 0;
}

Native C++ style typedef is replaced with .NET Action delegate. Additional parameter this is added to setCallback call, it is required to define the class instance which contains the callback function.

Collectives™ on Stack Overflow

C++/CLI managed class member callback by managed code

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related