[c#] What are the differences between delegates and events?

To understand the differences you can look at this 2 examples

Example with Delegates (in this case, an Action - that is a kind of delegate that doesn't return a value)

public class Animal
{
    public Action Run {get; set;}

    public void RaiseEvent()
    {
        if (Run != null)
        {
            Run();
        }
    }
}

To use the delegate, you should do something like this:

Animal animal= new Animal();
animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running") ;
animal.RaiseEvent();

This code works well but you could have some weak spots.

For example, if I write this:

animal.Run += () => Console.WriteLine("I'm running");
animal.Run += () => Console.WriteLine("I'm still running");
animal.Run = () => Console.WriteLine("I'm sleeping") ;

with the last line of code, I have overridden the previous behaviors just with one missing + (I have used = instead of +=)

Another weak spot is that every class which uses your Animal class can raise RaiseEvent just calling it animal.RaiseEvent().

To avoid these weak spots you can use events in c#.

Your Animal class will change in this way:

public class ArgsSpecial : EventArgs
{
    public ArgsSpecial (string val)
    {
        Operation=val;
    }

    public string Operation {get; set;}
} 

public class Animal
{
    // Empty delegate. In this way you are sure that value is always != null 
    // because no one outside of the class can change it.
    public event EventHandler<ArgsSpecial> Run = delegate{} 

    public void RaiseEvent()
    {  
         Run(this, new ArgsSpecial("Run faster"));
    }
}

to call events

 Animal animal= new Animal();
 animal.Run += (sender, e) => Console.WriteLine("I'm running. My value is {0}", e.Operation);
 animal.RaiseEvent();

Differences:

  1. You aren't using a public property but a public field (using events, the compiler protects your fields from unwanted access)
  2. Events can't be assigned directly. In this case, it won't give rise to the previous error that I have showed with overriding the behavior.
  3. No one outside of your class can raise the event.
  4. Events can be included in an interface declaration, whereas a field cannot

Notes:

EventHandler is declared as the following delegate:

public delegate void EventHandler (object sender, EventArgs e)

it takes a sender (of Object type) and event arguments. The sender is null if it comes from static methods.

This example, which uses EventHandler<ArgsSpecial>, can also be written using EventHandler instead.

Refer here for documentation about EventHandler

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