+= operator in Multicast Delegate

The question is not so trivial. If you look at the delegate instance dd you will find that this is an instance of some… class. This class actually encapsulate a lot of functionality. In particular, it has the container of handlers called "invocation list", accessible through dd.GetInvocationList(). This list is use to call all handlers in sequence as a result of invocation and pass the them their own instances of "self" (passed when a handler is added) and calling parameters passed from the invocation call.

The handler is added to the invocation list by "+" (for the first time) and "+=". Is it really so? Not exactly! The delegate instance looses its referential integrity on each "+="!

static void ReportDelegateEquivalence(Delegate left, Delegate right) {
    FormatEqual formatEqual = delegate(bool value) { if (value) return string.Empty; else return "NOT"; };
    System.Console.WriteLine(
        "Delegates are {0} equal, they are referentually {1} equal",
        formatEqual(left == right),
        formatEqual(object.ReferenceEquals(left, right)));
} //ReportDelegateEquivalence

static void TestDelegateEquivalence() {
    TestDelegate delegate1 = delegate(string name) { return 1; };
    TestDelegate delegate2 = delegate(string name) { return 1; };
    //output: "Delegates are NOT equal, they are referentually NOT equal":
    ReportDelegateEquivalence(delegate1, delegate2);
    
    delegate1 = delegate2;
    //output: "Delegates are equal, they are referentually equal":
    ReportDelegateEquivalence(delegate1, delegate2);
  
    delegate2 += delegate(string name) { return 2; };
    //output: "Delegates are equal, they are referentually equal":     
    ReportDelegateEquivalence(delegate1, delegate2);
} //TestDelegateEquivalence

This test shows that the delegate instance does not actually add a new handler. Instead, a brand new delegate instance is created with extended invocation list. So, the delegate instance is immutable. The purpose is the behavior is improving multithreading support: what is one thread is through the call of invocation while another thread is performing "+="?

Credit to Nishant Sivakumar how explained the purpose of this feature to me.

[EDIT]
Please see my recent article on the topic: Dynamic Method Dispatcher[^]. In particular, see this chapter: Dynamic Method Dispatcher[^].

—SA