private class Foo
{
    private event EventHandler<string>? PreEvent;
 
    private event EventHandler<string>? PostEvent;
 
    private object _lock = new();
 
    public event EventHandler<string> OnEvent
    {
        add
        {
            lock (_lock)
            {
                PreEvent += value;
                PostEvent += value;
            }
        }
        remove
        {
            lock (_lock)
            {
                PreEvent -= value;
                PostEvent -= value;
            }
        }
    }
 
    public void RaiseEvent()
    {
        PreEvent?.Invoke(this, $"{nameof(PreEvent)}");
 
        Console.WriteLine($"{nameof(Foo)} event raised");
 
        PostEvent?.Invoke(this, $"{nameof(PostEvent)}");
    }
}
 
private class Bar
{
    public Bar(Foo foo)
    {
        foo.OnEvent += Foo_OnEvent;
    }
 
    private void Foo_OnEvent(object? sender, string e)
    {
        Console.WriteLine($"{nameof(Bar)} receives {nameof(Foo)} event : {e}");
    }
}
 
private static void Main()
{
    Foo foo = new();
    Bar bar = new(foo);
 
    foo.RaiseEvent();
}
 
/* output:
Bar receives Foo event : PreEvent
Foo event raised
Bar receives Foo event : PostEvent
*/