使用接口事件引发继承

Question

for the following code:

delegate void deffault ();

public interface IBoom {
    event deffault OnBoom; 
}

public class BoomObject : IBoom {

    public event deffault OnBoom;

    public virtual void Start () {

    }

}

public class Grenade : BoomObject {

    public override void Start () {
        if (OnBoom != null)
            OnBoom ();
    }

}

I'm trying to Invoke an event in the base class throw a sub class, I don't know why it throw exception or if this is a good practice or not.

the exception I get is :

The event BoomObject.OnBoom' can only appear on the left hand side of += or -= when used outside of the type BoomObject'

Answer 1

This is not an exception, this is a compiler error.

The problem is that events implemented by public event DelegateType Event; can only be raised in the class that defines the event, not any derived class.

If you want to make this event accessible to derived classes, implement a protected RaiseEvent() method in your case class:

public class BoomObject : IBoom {
    public event deffault OnBoom;

    protected void RaiseBoom() {
         if (OnBoom != null)
             OnBoom ();
    }

    public virtual void Start () {
    }
}

public class Grenade : BoomObject {
    public override void Start () {
        RaiseBoom();
    }
}

Answer 2

You're not getting an exception but a syntax error from the compiler. Let us see what it says

The event BoomObject.OnBoom' can only appear on the left hand side of += or -= when used outside of the typeBoomObject

Okay so it tells you that you can only use -= or += for some reason. And that reason is that you are trying something outside the class where the event was declared. That is on purpose. Events can only be raised within the declaring class.

So to raise the event you have to eg create a method in the base class and call it in the derived class.