How to override an event handler that is not marked as virtual?

Question

I have to create a class that inherit from FileSystemWatcher. I should Intercept the event OnCreated and create my own event OnCreated. I have tried as follow:

public class LocalFolderWatcher : FileSystemWatcher, IWatcher
{
    ...
    public event LocalFolderEventHandler Created;

    protected override void OnCreated(FileSystemEventArgs e)
    {
        string path = System.IO.Path.GetDirectoryName(e.FullPath);
        LocalFolderEventArgs args = new LocalFolderEventArgs(e.ChangeType, this._gatewayConfigurationName, path, this._folderConfigurationName, e.Name);

        if (Created != null && base.EnableRaisingEvents && base.Path != null)
            Created(this, args);

        base.OnCreated(e);
    }
}

But I obtain an error: I cannot override a method that is not marked as virtual, abstract or override. I've tried to replace "override" with "new" but in this way the event is never raised..

How can I intercept the real "OnCreated" and replace with mine?

Thank you

Answer 1

You don't need to replace OnCreated, just create your own handler and pass it to the Created event.

public class LocalFolderWatcher : FileSystemWatcher, IWatcher
{
    //...
    public event LocalFolderEventHandler CustomCreated;

    public LocalFolderWatcher()
    {
        Created += OnCreated;
    }

    private new void OnCreated(object sender, FileSystemEventArgs e)
    {
        string path = System.IO.Path.GetDirectoryName(e.FullPath);
        LocalFolderEventArgs args = new LocalFolderEventArgs(e.ChangeType, this._gatewayConfigurationName, path, this._folderConfigurationName, e.Name);

        if (CustomCreated != null && base.EnableRaisingEvents && base.Path != null)
            CustomCreated(this, args);
    }
}

Answer 2

You can't.

If you're trying to modify what the default handler does, forget it, you can't. If you want to add your own behaviour, then you can just create a new handler and subscribe the event.

Answer 3

You could subscribe to the OnCreated event in FileSystemWatcher and then call your own method

Answer 4

Suppose we have the scenario

class BaseClass { public void M() { Console.WriteLine("BaseClass"); } }
class SubClass { public new void M() { Console.WriteLine("SubClass"); } }

void Main() {
    var x = new SubClass();
    x.M(); // Prints "SubClass"
    ((BaseClass)x).M(); // Prints "BaseClass"
}

In the first case, it calls SubClass.M and in the second it calls BaseClass.M. This is what "new" means - it creates a new method.

However, if we made BaseClass.M virtual and marked SubClass.M as override, then both of them would print "SubClass", because virtual calls check the runtime type of the caller. This explains why your event is never raised.

As IllidanS4 recommends, the best way is to add a listener to FileSystemWatcher.Created and make it call LocalFolderWatcher.