/// <summary>
 /// Validate the object.
 /// </summary>
 /// <exception cref="ValidationException">
 /// Thrown if validation fails
 /// </exception>
 public virtual void Validate()
 {
     if (Title == null)
     {
         throw new ValidationException(ValidationRules.CannotBeNull, "Title");
     }
     if (MaterialType != null)
     {
         MaterialType.Validate();
     }
 }