Tone's Notes

What happens if you add a delegate with an event more than once? The delegate will be invoked as many times as it is registered when the event “occurs”.

What happens if you remove a delegate from an event after adding it more than once? One registration is removed at a time.

What happens if you remove a delegate from an event if it has never been registered or if it has already been removed? Nothing.

Since delegates are equal so long as the wrapped methods are the same, it is not necessary to save the actual delegate instance that was added if you intend to remove it later.

        public class C1 {

            public event EventHandler E1;

            public void InvokeE1() {

                if (E1 != null) E1(this, new EventArgs());

            }

        }

        int invocations = 0;

        public void h(object sender, EventArgs args) {

            invocations++;

        }

        [TestMethod]

        public void TestRepeatedEventHandlerAddRemove1() {

            C1 c1 = new C1();

            c1.E1 += new EventHandler(h);

            Assert.AreEqual(0, invocations);

            c1.InvokeE1();

            Assert.AreEqual(1, invocations);

            // Verify that adding an event handler is invoked as many times as it is added.

            c1.E1 += new EventHandler(h);

            c1.InvokeE1();

            Assert.AreEqual(3, invocations);

            // Verify that removing an event handler removes only one copy at a time.

            c1.E1 -= new EventHandler(h);

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

            c1.E1 -= new EventHandler(h);

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

            // Verify that removing a non-existant event handler does nothing.

            c1.E1 -= new EventHandler(h);

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

        }

        [TestMethod]

        public void TestRepeatedEventHandlerAddRemove2() {

            // Verify that it doesn't matter whether the identical delegate instance is

            // used instead of just an "equal" delegate.

            C1 c1 = new C1();

            EventHandler eh = new EventHandler(h);

            c1.E1 += eh;

            Assert.AreEqual(0, invocations);

            c1.InvokeE1();

            Assert.AreEqual(1, invocations);

            c1.E1 += eh;

            c1.InvokeE1();

            Assert.AreEqual(3, invocations);

            c1.E1 -= eh;

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

            c1.E1 -= eh;

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

            c1.E1 -= eh;

            c1.InvokeE1();

            Assert.AreEqual(4, invocations);

        }