public class Training
{
    public Training()
    {
    }

    private List<Exercise> _exercises { get; set; }

    public void AddExercise(Exercise exercise)
    {
        if(_exercises == null)
            _exercises =new List<Exercise>();_exercises.Add(exercise);
    }

    public void AddExercises(IEnumerable<Exercise> exercises)
    {
        foreach (var exercise in exercises)
        {
            AddExercise(exercise);
        }
    }
}