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How to: Implement Managed Virtual Events

This sample implements virtual, managed events in an interface and class.

Example

// mcppv2_events5.cpp
// compile with: /clr
using namespace System;

public delegate void MyDel();
public delegate int MyDel2(int, float);

// managed class with virtual event
ref class IEFace {
public:
   virtual event MyDel ^ E;   // declares three accessors (add, remove, and raise)
};

// managed interface with virtual event
public interface struct IEFace2 {
public:
   event MyDel2 ^ E2;   // declares two accessors (add and remove)
};

// implement virtual events
ref class EventSource : public IEFace, public IEFace2 {
public:
   virtual event MyDel2 ^ E2;

   void Fire_E() {
      E();
   }

   int Fire_E2(int i, float f) {
      try {
         return E2(i, f);
      }
      catch(System::NullReferenceException^) {
         return 0;   // no handlers
      }
   }
};

// class to hold event handlers, the event receiver
public ref struct EventReceiver {
   // first handler
   void H1() {
      Console::WriteLine("In handler H1");
   }

   // second handler
   int H2(int i, float f) {
      Console::WriteLine("In handler H2 with args {0} and {1}", i.ToString(), f.ToString());
      return 0;
   }
};

int main() {
   EventSource ^ pE = gcnew EventSource;
   EventReceiver ^ pR = gcnew EventReceiver;

   // add event handlers
   pE->E += gcnew MyDel(pR, &EventReceiver::H1);
   pE->E2 += gcnew MyDel2(pR, &EventReceiver::H2);

   // raise events
   pE->Fire_E();
   pE->Fire_E2(1, 2.2);

   // remove event handlers
   pE->E -= gcnew MyDel(pR, &EventReceiver::H1);
   pE->E2 -= gcnew MyDel2(pR, &EventReceiver::H2);

   // raise events, but no handlers so, no effect
   pE->Fire_E();
   pE->Fire_E2(1, 2.5);
}

Output

In handler H1
In handler H2 with args 1 and 2.2

Example

Note that you are currently not able to specify that a simple event overrides or hides a base class event. You must define all of the event's accessor functions, and specify the new or override keyword on each accessor function.

// mcppv2_events5_a.cpp
// compile with: /clr /c
delegate void Del();

ref struct A {
   virtual event Del ^E;
   virtual event Del ^E2;
};

ref struct B : A {
   virtual event Del ^E override;   // C3797
   virtual event Del ^E2 new;   // C3797
};

ref struct C : B {
   virtual event Del ^E {   // OK
      void raise() override {}
      void add(Del ^) override {}
      void remove(Del^) override {}
   }

   virtual event Del ^E2 {   // OK
      void raise() new {}
      void add(Del ^) new {}
      void remove(Del^) new {}
   }
};

See Also

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