var worker = new Worker();
var anotherClass = new AnotherClass();
worker.WorkDone += anotherClass.WorkDone;
worker.DoWork();

class Worker
{
    public event Action WorkDone;

    public void DoWork()
    {
        //..
        var workDoneCopy = WorkDone;

        if (workDoneCopy != null)
            workDoneCopy();
    }
}

class AnotherClass
{
    public void WorkDone()
    {
        //..
    }
}