Since we have multicast delegates, why do we need events?

Question

I was asked this question in an interview, and either I'm suffering from brain-lock or just plain dumb, but I didn't have an answer.

If I remember correctly, events are implemented using Multicast delegates. However, the delegate is usually private (so that consumers can't play on it directly). Events on the other hand have a public API — Flydog57, Dec 17 '19 at 20:00
Per [msdn](https://learn.microsoft.com/en-us/dotnet/api/system.multicastdelegate?view=netframework-4.8) _Compilers and other tools can derive from this class, but you cannot derive from it explicitly. The same is true of the `Delegate` class._ — Pavel Anikhouski, Dec 17 '19 at 20:10
Events are used to *restrict* access to a delegate. Just like properties restrict access to a regular variable. Properties permit get and set, events permit add and remove. Important, this ensures that external code cannot make event subscriptions from other code disappear with a single assignment. They mostly look different because you don't often actually write the add and remove accessors. The ones that the compiler auto-generates are often good enough. — Hans Passant, Dec 17 '19 at 20:12
you're not the only one https://stackoverflow.com/questions/563549/difference-between-events-and-delegates-and-its-respective-applications — Nikki9696, Dec 17 '19 at 20:30
also see https://learn.microsoft.com/en-us/dotnet/csharp/distinguish-delegates-events — Nikki9696, Dec 17 '19 at 20:34
Can you answer the question "since we have fields, why do we need properties?" If you can answer that question, you can make a good start on the question you asked. If you cannot answer that question, work on that one first! — Eric Lippert, Dec 17 '19 at 21:40
Hi Grv_9098, did the answer help answer your question. If yes please consider accepting it by clicking the check-mark. This will also help others understand you've found a solution — Clint, Dec 19 '19 at 13:34

Clint · Accepted Answer · 2019-12-19T13:31:30.877

Couple of reasons why we need events:

Restricting scope, you do not want to expose your events, like you can for your delegates.
You can have events as fields in your interfaces and not delegates

Example below:

Class Printer
{
    public event EventHandler Print;

    public void Start()
    {
        OnPrint();
    }

    protected virtual void OnPrint()
    {
        Print?.Invoke(this,EventArgs.Empty);
    }
}
class Program
{
    static void Main(string[] args)
    {
        //When Print is an EventHander
        var printer = new Printer();
        printer.Print += PrintEvent;
        printer.Start();

        //If Print was a delegate this is possible, else you get compile time errors 
        printer.Print(null,null); // Events will not allow to have a direct invoke 
        printer.Print = null; //You cannot assign a null to an Event Handler
    }
    private static void PrintEvent(object sender, EventArgs e)
    {
        System.Console.WriteLine("Printing event");
    }
}

Since we have multicast delegates, why do we need events?

1 Answers1