How do you trigger an event across classes?

Question

I am writing a Class Library that will be used by other applications. I am writing it in C#.NET. I am having a problem with triggering events across classes. Here is what I need to do...

public class ClassLibrary
{
    public event EventHandler DeviceAttached;

    public ClassLibrary()
    {
        // do some stuff
        OtherClass.Start();
    }
}

public class OtherClass : Form
{
    public Start()
    {
        // do things here to initialize receiving messages
    }

    protected override void WndProc (ref message m)
    {
       if (....)
       {
          // THIS IS WHERE I WANT TO TRIGGER THE DEVICE ATTACHED EVENT IN ClassLibrary
          // I can't seem to access the eventhandler here to trigger it.
          // How do I do it?

       }
       base.WndProc(ref m);
    }

}

Then in the application that is using the class library I will do this...

public class ClientApplication
{
    void main()
    {
       ClassLibrary myCL = new ClassLibrary();
       myCL.DeviceAttached += new EventHandler(myCl_deviceAttached);
    }

    void myCl_deviceAttached(object sender, EventArgs e)
    {
         //do stuff...
    }
}

Answer 1

You cannot do this. Events can only be raised from within the class that declares the event.

Typically, you'd add a method on your class to raise the event, and call the method:

public class ClassLibrary 
{ 
    public event EventHandler DeviceAttached; 
    public void NotifyDeviceAttached()
    {
       // Do processing and raise event
     }

Then, in your other code, you'd just call myCL.NotifyDeviceAttached();

Answer 2

Event handlers can only be called directly by the class that declared them. If you need to call ClassLibrary.DeviceAttached from outside that class, you need to add a utility method like the following:

public void OnDeviceAttached()
{
    DeviceAttached();
}

Answer 3

You may not want to use an event at all here. This is an oversimplification, but generally an event is something raised by a child component when it needs to communicate something back to its parent. In your case (I'm inferring from your code) your form is listening for a specific message (when a device is attached?), and when it spots that message it needs to tell myCL about it. For this purpose, you would instead just create a method in ClassLibrary and call it from your form.

Answer 4

I think you should change your perspective on how events work. OtherClass should "own" the event and trigger it. ClassLibrary or ClientApplication (whichever you choose) "listens" to the event by "subscribing" to it and does a certain action when this event occurs.

How to implement this:

public class ClassLibrary
{
    public OtherClass myOtherCl;

    public ClassLibrary()
    {
        myOtherCl= new OtherClass();
        myOtherCl.Start();
    }
}

Trigger the event in the class where it logically happens, where it is detected.

public class OtherClass : Form { public event EventHandler DeviceAttached;

public Start()
{
    // do things here to initialize receiving messages
}

protected override void WndProc (ref message m)
{
   if (....)
   {
      OnDeviceAttach();
   }
   base.WndProc(ref m);
}

    public void OnDeviceAttach()
    {
        if (DeviceAttached != null)
            DeviceAttached ();
    }

}

Finally, whoever needs to listen to the event needs access to the instance of the class holding the event, that is why myOtherCl was made public in this example.

public class ClientApplication
{
    void main()
    {
       ClassLibrary myCL = new ClassLibrary();
       myCL.myOtherCl.DeviceAttached += new EventHandler(myCl_deviceAttached);
    }

    void myCl_deviceAttached(object sender, EventArgs e)
    {
         //do stuff...
    }
}