How can I test that an event contains an event handler?

Question

I want to test that class A's RegisterEventHandlers() method registers one of its methods as an EventHandler for an event on class B. How can I do that? I'm using moq, if that matters.

• I don't think there's a way to inspect the event handler delegate from outside the class (please correct me if I'm wrong).
• It'd be nice if I could trigger the event and then assert that my callback was called, but if I mock the interface of the A class (and set up an expectation for the callback) then I lose the implementation of RegisterEventHandlers(), which is the method I'm testing in the first place.
• Mocking the B class's event would be the best option, but I don't see what method I'd have to intercept to do this. Is there some way to set up a mock for an event, and intercept the += method call?

Is there a clean solution to this?

Answer 1

You can get the invocation list for an event outside the class declaring the event - but it involves reflection. Below is a code example showing how you can determine which methods (on target instance a) are added to the event b.TheEvent after a call to a.RegisterEventHandlers(). Paste the code below in a code file and add to a form or console project: Test test = new Test(); test.Run();

using System;
using System.Reflection;
using System.Diagnostics;
using System.Collections.Generic;

   public class A
   {
      B m_b = new B();

      public void RegisterEventHandlers()
      {
         m_b.TheEvent += new EventHandler(Handler_TheEvent);
         m_b.TheEvent += new EventHandler(AnotherHandler_TheEvent);
      }

      public A()
      { 
         m_b.TheEvent += new EventHandler(InitialHandler_TheEvent);
      }

      void InitialHandler_TheEvent(object sender, EventArgs e)
      { }

      void Handler_TheEvent(object sender, EventArgs e)
      { }

      void AnotherHandler_TheEvent(object sender, EventArgs e)
      { }
   }

   public class B
   {
      public event EventHandler TheEvent;
      //{
      //   //Note that if we declared TheEvent without the add/remove methods, the
      //   //following would still generated internally and the underlying member
      //   //(here m_theEvent) can be accessed via Reflection. The automatically
      //   //generated version has a private field with the same name as the event
      //   //(i.e. "TheEvent")

      //   add { m_theEvent += value; }
      //   remove { m_theEvent -= value; }
      //}
      //EventHandler m_theEvent; //"TheEvent" if we don't implement add/remove


      //The following shows how the event can be invoked using the underlying multicast delegate.
      //We use this knowledge when invoking via reflection (of course, normally we just write
      //if (TheEvent != null) TheEvent(this, EventArgs.Empty)
      public void ExampleInvokeTheEvent()
      {
         Delegate[] dels = TheEvent.GetInvocationList();
         foreach (Delegate del in dels)
         {
            MethodInfo method = del.Method;
            //This does the same as ThisEvent(this, EventArgs.Empty) for a single registered target
            method.Invoke(this, new object[] { EventArgs.Empty });
         }
      }
   }


   public class Test
   {
      List<Delegate> FindRegisteredDelegates(A instanceRegisteringEvents, B instanceWithEventHandler, string sEventName)
      {
         A a = instanceRegisteringEvents;
         B b = instanceWithEventHandler;

         //Lets assume that we know that we are looking for a private instance field with name sEventName ("TheEvent"), 
         //i.e the event handler does not implement add/remove.
         //(otherwise we would need more reflection to determine what we are looking for)
         BindingFlags filter = BindingFlags.Instance | BindingFlags.NonPublic;

         //Lets assume that TheEvent does not implement the add and remove methods, in which case
         //the name of the relevant field is just the same as the event itself
         string sName = sEventName; //("TheEvent")

         FieldInfo fieldTheEvent = b.GetType().GetField(sName, filter);

         //The field that we get has type EventHandler and can be invoked as in ExampleInvokeTheEvent
         EventHandler eh = (EventHandler)fieldTheEvent.GetValue(b);

         //If the event handler is null then nobody has registered with it yet (just return an empty list)
         if (eh == null) return new List<Delegate>();


         List<Delegate> dels = new List<Delegate>(eh.GetInvocationList());

         //Only return those elements in the invokation list whose target is a.
         return dels.FindAll(delegate(Delegate del) { return Object.ReferenceEquals(del.Target, a); });
      }

      public void Run()
      {
         A a = new A();

         //We would need to check the set of delegates returned before we call this

         //Lets assume we know how to find the all instances of B that A has registered with
         //For know, lets assume there is just one in the field m_b of A.
         FieldInfo fieldB = a.GetType().GetField("m_b", BindingFlags.Instance | BindingFlags.NonPublic);
         B b = (B)fieldB.GetValue(a);

         //Now we can find out how many times a.RegisterEventHandlers is registered with b
         List<Delegate> delsBefore = FindRegisteredDelegates(a, b, "TheEvent");

         a.RegisterEventHandlers();

         List<Delegate> delsAfter = FindRegisteredDelegates(a, b, "TheEvent");

         List<Delegate> delsAdded = new List<Delegate>();
         foreach (Delegate delAfter in delsAfter)
         {
            bool inBefore = false;
            foreach (Delegate delBefore in delsBefore)
            {
               if ((delBefore.Method == delAfter.Method)
                  && (Object.ReferenceEquals(delBefore.Target, delAfter.Target)))
               {
                  //NOTE: The check for Object.ReferenceEquals(delBefore.Target, delAfter.Target) above is not necessary 
                  //     here since we defined FindRegisteredDelegates to only return those for which .Taget == a)

                  inBefore = true;
                  break;
               }
            }
            if (!inBefore) delsAdded.Add(delAfter);
         }

         Debug.WriteLine("Handlers added to b.TheEvent in a.RegisterEventHandlers:");
         foreach (Delegate del in delsAdded)
         {
            Debug.WriteLine(del.Method.Name);
         }


      }
   }

Answer 2

When mocking B, declare the EventHandler like this:

public class B : IB
{
  public int EventsRegistered;
  public event EventHandler Junk
  {
     add
     {
        this.EventsRegistered++;
     }
     remove
     {
        this.EventsRegistered--;
     }
  }
}

I'm not certain that moq allows this, but I'm sure you can create your own mock class.