Add an new event to your control / class:
public event EventHandler<EventArgs> MyTextChanged;
In textBox_TextChanged
do this:
if (MyTextChanged != null)
{
MyTextChanged(this, EventArgs.Empty);
}
This can than be enhanced with your custom event args if you would like, etc.
If just named it MyTextChanged to mark what the new event is. You of course can name it TextChanged.