Use event and delegate in subclass

问题

Why cant i use the event declared in Base from Sub?

class Program
{
    static void Main(string[] args)
    {
        Sub sub = new Sub();
        sub.log += new Base.logEvent(sub_log);
        sub.go();
    }

    static void sub_log(string message, int level)
    {
        Console.Out.WriteLine(message + " " + level);
    }
}

public abstract class Base
{
    public delegate void logEvent(String message, int level);

    public event logEvent log;
}

public class Sub : Base
{

    public void go()
    {
        log("Test", 1); // <-- this wont compile
    }
}

回答1:

Events may only be invoked from the class that declares them.

From outside of the definition of a class (even in a derived class) you can only register and unregister from an event. Inside of the class, the compiler only allows you to raise the event. This is a by-design behavior of C# (which actually changes slightly in C#4 - Chris Burrows describes the changes on his blog).

What you want to do here is provide a RaiseLogEvent() method in the base class, which would allow the derived class to invoke this event.

public abstract class Base
{ 
  public delegate void logEvent(String message, int level); 

  public event logEvent log; 

  protected void RaiseLogEvent( string msg, int level )
  {
      // note the idomatic use of the copy/test/invoke pattern...
      logEvent evt = log;
      if( evt != null )
      {
          evt( msg, level );
      }
  }
}

As an aside, you should consider using the EventHandler<> delegate type, rather than creating your own event types when possible.

回答2:

Because events can only be called from the declaring class. Just create a method in the base class to call it:

protected virtual RaiseLogEvent(string s, int i)
{
  log(s, i);
}

So you can use it in deriving classes, and even override it.

On another note, I would strongly advise you to follow the design guidelines for events, and create an own EventArgs class, and use the EventHandler<T> delegate.

回答3:

Yes, you can declare events in a base class so that they can also be raised from derived classes.

Create a protected invoking method for the event. By calling this invoking method, derived classes can invoke the event.

Read the below standard way to declare events in a base class so that they can also be raised from derived classes:

Raise Base Class Events in Derived Classes

来源：https://stackoverflow.com/questions/3064846/use-event-and-delegate-in-subclass

标签

events

delegates