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Correct event invocation

Be aware that if there are no subscribers a .NET event will be null. Therefore when raising the event from C# test it for null first.

    public event EventHandler SelectedNodeChanged;
 
    protected virtual void OnSelectedNodeChanged(object sender, EventArgs e)
    {
        //Event will be null if there are no subscribers
        if (SelectedNodeChanged != null)
        {
            SelectedNodeChanged(this, e);
        }
    }

However in multithreaded application the last subscriber can unsubscribe immediately after the null check and before the event is raised. To avoid a null reference exception make a temporary copy of the event.

    public event EventHandler SelectedNodeChanged;
 
    protected virtual void OnSelectedNodeChanged(object sender, EventArgs e)
    {
        //Make a temporary copy of the event to avoid possibility of
        //a race condition if the last subscriber unsubscribes
        //immediately after the null check and before the event is raised.
        EventHandler handler = SelectedNodeChanged;
 
        //Event will be null if there are no subscribers
        if (handler != null)
        {
            handler(this, e);
        }
    }