C# Delegate Instantiation - delegates

Question

class Program
{
delegate void StringProcessor(string input);
class Person
{
string name;
public Person(string name) { this.name = name; }
public void Say(string message)
{
Console.WriteLine("{0} says: {1}", name, message);
}
}
class Background
{
public static void Note(string note)
{
Console.WriteLine("({0})", note);
}
}
static void Main(string[] args)
{
Person jon = new Person("Jon");
Person tom = new Person("Tom");
StringProcessor jonsVoice, tomsVoice, background;
jonsVoice = new StringProcessor(jon.Say);
tomsVoice = new StringProcessor(tom.Say);
background = new StringProcessor(Background.Note);
StringProcessor p = new StringProcessor(jonsVoice);
p += tomsVoice;
p -= jonsVoice;
p("Hello");
}
}
This program prints
Jon says: Hello
Tom says: Hello
instead of
Tom says: Hello
This removal doesn't work: p -= jonsVoice;
But why?
Could you please clarify this or get some useful links about delegate instantiation with another delegate. I'm interested in undercover information.

Answer 1

A delegate is an object which represents a method and, optionally, the "this" object associated with that method.
When you combine delegates using the += operator, the result is a Multicast Delegate that represents a list of delegates to be invoked. (In fact all delegates are Multicast delegates, creating a delegate like this:
jonsVoice = new StringProcessor(jon.Say);
results in a Multicast Delegate with a single delegate in it's InvocationList).
You can remove delegates from the Invocation list by using the -= operator.
So why didn't your example above work? Because there is no delegate matching jonsVoice in the invocation list of p.
p has an invocation list that consists of two delegates:
one pointing at the Say method of the tomObject
one pointing at the Invoke method of a StringProcessor object
That is because you initialised p by passing the StringProcessor constructor another StringProcessor:
jonsVoice = new StringProcessor(jon.Say);
...
StringProcessor p = new StringProcessor(jonsVoice);
Resulting in a StringProcessor delegate that points at another StringProcessor delegate.
You can examine the invocation list of a delegate by calling the GetInvocationList method on that delegate. This will return an array of delegates in the order they would be invoked.
StringProcessor p = new StringProcessor(jonsVoice);
p += tomsVoice;
p -= jonsVoice;
foreach (var delegateItem in p.GetInvocationList())
{
Console.WriteLine("Delegate of {0}.{1}", delegateItem.Target.GetType().Name, delegateItem.Method.Name);
}
Which results in:
Delegate of StringProcessor.Invoke
Delegate of Person.Say
When you call p -= jonsVoice, the invocation list does not contain a delegate that points at the Person.Say method of the jon objects (like jonsVoice does) and so there is nothing to remove.
If you initialize p like this instead:
StringProcessor p = jonsVoice;
You should get the results you expect.