How to: Use Events in C++/CLI

This article shows how to use an interface that declares an event and a function to invoke that event, and the class and event handler that implement the interface.

The following code example adds an event handler, invokes the event—which causes the event handler to write its name to the console—and then removes the event handler.

// mcppv2_events2.cpp
// compile with: /clr
using namespace System;

delegate void Del(int, float);

// interface that has an event and a function to invoke the event
interface struct I {
public:
   event Del ^ E;
   void fire(int, float);   
};

// class that implements the interface event and function
ref class EventSource: public I {
public:
   virtual event Del^ E;
   virtual void fire(int i, float f) {
      E(i, f);
   }
};

// class that defines the event handler
ref class EventReceiver {
public:
   void Handler(int i , float f) {
      Console::WriteLine("EventReceiver::Handler");
   }
};

int main () {
   I^ es = gcnew EventSource();
   EventReceiver^ er = gcnew EventReceiver();

   // hook the handler to the event
   es->E += gcnew Del(er, &EventReceiver::Handler);

   // call the event
   es -> fire(1, 3.14);

   // unhook the handler from the event
   es->E -= gcnew Del(er, &EventReceiver::Handler);
}

Output

EventReceiver::Handler

The following sample shows how to define an event's behavior when handlers are added or removed, and when an event is raised.

// mcppv2_events6.cpp
// compile with: /clr
using namespace System;

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

ref class EventSource {
public:
   MyDel ^ pE;
   MyDel2 ^ pE2;

   event MyDel^ E {
      void add(MyDel^ p) {
         pE = static_cast<MyDel^> (Delegate::Combine(pE, p)); 
         // cannot refer directly to the event
         // E = static_cast<MyDel^> (Delegate::Combine(pE, p));   // error
      }

      void remove(MyDel^ p) {
         pE = static_cast<MyDel^> (Delegate::Remove(pE, p));
      }

      void raise() {
         if (pE != nullptr)
            pE->Invoke();
      }
   }  // E event block
   
   event MyDel2^ E2 {
      void add(MyDel2^ p2) {
         pE2 = static_cast<MyDel2^> (Delegate::Combine(pE2, p2)); 
      }

      void remove(MyDel2^ p2) {
         pE2 = static_cast<MyDel2^> (Delegate::Remove(pE2, p2));
      }

      int raise(int i, float f) {
         if (pE2 != nullptr) {
            return pE2->Invoke(i, f);
         }
         return 1;
      }
   } // E2 event block
};

public ref struct EventReceiver {
   void H1() {
      Console::WriteLine("In event handler H1");
   }

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

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

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

   // raise events
   pE->E();
   pE->E2::raise(1, 2.2);   // call event through scope path

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

   // raise events, but no handlers
   pE->E();
   pE->E2::raise(1, 2.5);
}

Output

In event handler H1
In event handler H2 with args 1 and 2.2

This sample shows how to override the default access on the add, remove, and raise events methods:

// mcppv2_events3.cpp
// compile with: /clr
public delegate void f(int);

public ref struct E {
   f ^ _E;
public:
   void handler(int i) {
      System::Console::WriteLine(i);
   }

   E() {
      _E = nullptr;
   }

   event f^ Event {
      void add(f ^ d) {
         _E += d;
      }
   private:
      void remove(f ^ d) {
        _E -= d;
      }

   protected:
      void raise(int i) {
         if (_E) {
            _E->Invoke(i);
         }
      }
   }

   // a member function to access all event methods
   static void Go() {
      E^ pE = gcnew E;
      pE->Event += gcnew f(pE, &E::handler);
      pE->Event(17);   // prints 17
      pE->Event -= gcnew f(pE, &E::handler);
      pE->Event(17);   // no output
   }
};

int main() {
   E::Go();
}

Output

17

An event receiver, or any other client code, can add one or more handlers to an event.

// mcppv2_events4.cpp
// compile with: /clr
using namespace System;
#include <stdio.h>

delegate void ClickEventHandler(int, double);
delegate void DblClickEventHandler(String^);

ref class EventSource {
public:
   event ClickEventHandler^ OnClick;
   event DblClickEventHandler^ OnDblClick;

   void FireEvents() {
      OnClick(7, 3.14159);
      OnDblClick("Started");
   }
};

ref struct EventReceiver {
public:
   void Handler1(int x, double y) {
      System::Console::Write("Click(x={0},y={1})\n", x, y);
   };

   void Handler2(String^ s) {
      System::Console::Write("DblClick(s={0})\n", s);
   }

   void Handler3(String^ s) {
      System::Console::WriteLine("DblClickAgain(s={0})\n", s);
   }

   void AddHandlers(EventSource^ pES) {
      pES->OnClick += 
         gcnew ClickEventHandler(this,&EventReceiver::Handler1);
      pES->OnDblClick += 
         gcnew DblClickEventHandler(this,&EventReceiver::Handler2);
      pES->OnDblClick += 
         gcnew DblClickEventHandler(this, &EventReceiver::Handler3);
   }

   void RemoveHandlers(EventSource^ pES) {
      pES->OnClick -= 
         gcnew ClickEventHandler(this, &EventReceiver::Handler1);
      pES->OnDblClick -= 
         gcnew DblClickEventHandler(this, &EventReceiver::Handler2);
      pES->OnDblClick -= 
         gcnew DblClickEventHandler(this, &EventReceiver::Handler3);
   }
};

int main() {
   EventSource^ pES = gcnew EventSource;
   EventReceiver^ pER = gcnew EventReceiver;

   // add handlers
   pER->AddHandlers(pES);
   
   pES->FireEvents();

   // remove handlers
   pER->RemoveHandlers(pES);
}

Output

Click(x=7,y=3.14159)
DblClick(s=System.Char[])
DblClickAgain(s=System.Char[])

The following sample shows how to define and use static events.

// mcppv2_events7.cpp
// compile with: /clr
using namespace System;

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

ref class EventSource {
public:
   static MyDel ^ psE;
   static event MyDel2 ^ E2;   // event keyword, compiler generates add, 
                               // remove, and Invoke

   static event MyDel ^ E {
      static void add(MyDel ^ p) {
         psE = static_cast<MyDel^> (Delegate::Combine(psE, p));
      }

      static void remove(MyDel^ p) {
         psE = static_cast<MyDel^> (Delegate::Remove(psE, p));
      }

      static void raise() {
         if (psE != nullptr)   //psE!=0 -> C2679, use nullptr
            psE->Invoke(); 
      }
   }

   static int Fire_E2(int i, float f) {
      return E2(i, f);
   }
};

public ref struct EventReceiver {
   void H1() {
      Console::WriteLine("In event handler H1");
   }

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

int main() {
   EventSource^ pE = gcnew EventSource;
   EventReceiver^ pR = gcnew EventReceiver;
   
   // Called with "this"
   // hook event handlers
   pE->E += gcnew MyDel(pR, &EventReceiver::H1);
   pE->E2 += gcnew MyDel2(pR, &EventReceiver::H2);

   // raise events
   pE->E();
   pE->Fire_E2(11, 11.11);

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

   // Not called with "this"
   // hook event handler
   EventSource::E += gcnew MyDel(pR, &EventReceiver::H1);
   EventSource::E2 += gcnew MyDel2(pR, &EventReceiver::H2);

   // raise events
   EventSource::E();
   EventSource::Fire_E2(22, 22.22);

   // unhook event handlers
   EventSource::E -= gcnew MyDel(pR, &EventReceiver::H1);
   EventSource::E2 -= gcnew MyDel2(pR, &EventReceiver::H2);
}

Output

In event handler H1
In event handler H2 with args 11 and 11.11
In event handler H1
In event handler H2 with args 22 and 22.22

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

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

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

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

// managed interface that has a 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

A simple event cannot be specified to override or hide a base class event. You must define all of the event's accessor functions, and then 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 {}
   }
};

The following sample shows how to implement an abstract event.

// mcppv2_events10.cpp
// compile with: /clr /W1
using namespace System;
public delegate void Del();
public delegate void Del2(String^ s);

interface struct IEvent {
public:
   // in this case, no raised method is defined
   event Del^ Event1;

   event Del2^ Event2 {
   public:
      void add(Del2^ _d);
      void remove(Del2^ _d);
      void raise(String^ s);
   }

   void fire();
};

ref class EventSource: public IEvent {
public:
   virtual event Del^ Event1;
   event Del2^ Event2 {
      virtual void add(Del2^ _d) {
         d = safe_cast<Del2^>(System::Delegate::Combine(d, _d));
      }

      virtual void remove(Del2^ _d) {
         d = safe_cast<Del2^>(System::Delegate::Remove(d, _d));
      }

      virtual void raise(String^ s) {
         if (d) {
            d->Invoke(s);
         }
      }
   }

   virtual void fire() {
      return Event1();
   }

private:
   Del2^ d;
};

ref class EventReceiver {
public:
   void func() {
      Console::WriteLine("hi");
   }

   void func(String^ str) {
      Console::WriteLine(str);
   }
};

int main () {
   IEvent^ es = gcnew EventSource;
   EventReceiver^ er = gcnew EventReceiver;
   es->Event1 += gcnew Del(er, &EventReceiver::func);
   es->Event2 += gcnew Del2(er, &EventReceiver::func);
   
   es->fire();
   es->Event2("hello from Event2");
   es->Event1 -= gcnew Del(er, &EventReceiver::func);
   es->Event2 -= gcnew Del2(er, &EventReceiver::func);
   es->Event2("hello from Event2");
}

Output

hi
hello from Event2

An event and event handler can be defined in one assembly, and consumed by another assembly.

// mcppv2_events8.cpp
// compile with: /LD /clr
using namespace System;

public delegate void Del(String^ s);

public ref class Source {
public:
   event Del^ Event;
   void Fire(String^ s) {
      Event(s);
   }
};

This client code consumes the event:

// mcppv2_events9.cpp
// compile with: /clr
#using "mcppv2_events8.dll"
using namespace System;

ref class Receiver {
public:
   void Handler(String^ s) {
      Console::WriteLine(s);
   }
};

int main() {
   Source^ src = gcnew Source;
   Receiver^ rc1 = gcnew Receiver;
   Receiver^ rc2 = gcnew Receiver;
   src -> Event += gcnew Del(rc1, &Receiver::Handler);
   src -> Event += gcnew Del(rc2, &Receiver::Handler);
   src->Fire("hello");
   src -> Event -= gcnew Del(rc1, &Receiver::Handler);
   src -> Event -= gcnew Del(rc2, &Receiver::Handler);
}

Output

hello
hello
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