Develop Reference

DDD modelling of articles, variants and their parameters

I am learning DDD and trying to model articles, its variants and parameters.
Article can be on it's own without variants
Variant must be child of an article
both article and variant can have some parameters (colors, brands, sizes...), physical quantities (width, length, some article-specific like inner length)
If you set some parameter on an article, it can be "synchronized" to it's children variants
you can override this in a variant by setting that parameter as "unlinked", then this variant would have different parameter value than article
some parameters can be set multiple times (color: red, blue), but some only once (brand)
those parameters are dynamically create, it's not a Color or Brand property but key-value selected from preconfigured values
I think my main aggregate roots will be Article and Variant.
My current code looks like this:
internal class Article : AggregateRoot<ArticleId>
{
private readonly ISet<VariantId> _variants = new HashSet<VariantId>();
private readonly ISet<AssignedParameter> _parameters = new HashSet<AssignedParameter>();
private readonly ISet<AssignedPhysicalQuantity> _physicalQuantities = new HashSet<AssignedPhysicalQuantity>();
public string Name { get; private set; }
public string Catalog { get; private set; }
public IReadOnlySet<VariantId> Variants => _variants.AsReadOnly();
public IReadOnlySet<AssignedParameter> Parameters => _parameters.AsReadOnly();
public IReadOnlySet<AssignedPhysicalQuantity> PhysicalQuantities => _physicalQuantities.AsReadOnly();
private Article(ArticleId id, string name, string catalog)
: base(id)
{
Name = name;
Catalog = catalog;
}
public static Article Register(ArticleId id, string name, string catalog)
{
var article = new Article(id, name, catalog);
article.AddEvent(new ArticleRegistered(article.Id, article.Name, article.Catalog));
return article;
}
public void AssignParameter(Parameter parameter, ParameterValue parameterValue, bool syncToVariants)
{
if (!parameter.CanBeAssignedMultipleTimes && _parameters.Any(p => p.ParameterId == parameter.Id))
{
throw new ParameterCanBeAssignedOnlyOnceException($"Parameter {parameter.Id} can by assigned only once.");
}
var assignedParameter = new AssignedParameter(parameter.Id, parameterValue.Id, syncToVariants);
if (!_parameters.Add(assignedParameter))
{
throw new ParameterIsAlreadyAssignedException($"Parameter {parameter.Id} with value {parameterValue.Id} is already assigned.");
}
AddEvent(new ArticleParameterAssigned(Id, assignedParameter.ParameterId, assignedParameter.ParameterValueId));
}
public void UnassignParameter(Parameter parameter, ParameterValue parameterValue)
{
var assignedParameter = _parameters.FirstOrDefault(p => p.ParameterId == parameter.Id && p.ParameterValueId == parameterValue.Id);
if (assignedParameter is null)
{
throw new ParameterIsNotAssignedException($"Parameter {parameter.Id} is not assigned.");
}
_parameters.Remove(assignedParameter);
AddEvent(new ArticleParameterUnassigned(Id, assignedParameter.ParameterId, assignedParameter.ParameterValueId));
}
// physical quantity assign / unassign are similar to parameters
}
internal class Variant : AggregateRoot<VariantId>
{
private readonly ISet<AssignedParameter> _parameters = new HashSet<AssignedParameter>();
private readonly ISet<AssignedPhysicalQuantity> _physicalQuantities = new HashSet<AssignedPhysicalQuantity>();
public string Name { get; private set; }
public string Catalog { get; private set; }
public EanCode Ean { get; private set; }
public decimal Weight { get; private set; }
public IReadOnlySet<AssignedParameter> Parameters => _parameters.AsReadOnly();
public IReadOnlySet<AssignedPhysicalQuantity> PhysicalQuantities => _physicalQuantities.AsReadOnly();
internal Variant(VariantId id, string name, string catalog, EanCode ean, decimal weight)
: base(id)
{
Name = name;
Catalog = catalog;
Ean = ean;
Weight = weight;
}
// parameter and physical quantity assignment methods
}
Parameters:
internal class Parameter : AggregateRoot<ParameterId>
{
private readonly ISet<ParameterValue> _values = new HashSet<ParameterValue>();
public string Code { get; private set; }
public string Name { get; private set; }
public bool CanBeAssignedMultipleTimes { get; private set; }
public IReadOnlySet<ParameterValue> Values => _values.AsReadOnly();
public Parameter(ParameterId id, string code, string name, bool canBeAssignedMultipleTimes)
: base(id)
{
Code = code;
Name = name;
CanBeAssignedMultipleTimes = canBeAssignedMultipleTimes;
}
}
internal class ParameterValue : Entity<ParameterValueId>
{
public string Code { get; private set; }
public string Name { get; private set; }
public Parameter Parameter { get; private init; } = null!;
public ParameterValue(ParameterValueId id, string code, string name)
: base(id)
{
Code = code;
Name = name;
}
}
Value objects:
// for Article, variant doesn't have SyncToVariants property and has some other
internal class AssignedParameter : ValueObject
{
public ParameterId ParameterId { get; private init; }
public ParameterValueId ParameterValueId { get; private init; }
public bool SyncToVariants { get; private init; }
public AssignedParameter(ParameterId parameterId, ParameterValueId parameterValueId, bool syncToVariants)
{
ParameterId = parameterId;
ParameterValueId = parameterValueId;
SyncToVariants = syncToVariants;
}
protected override IEnumerable<object> GetEqualityComponents()
{
yield return ParameterId;
yield return ParameterValueId;
}
}
internal class AssignedPhysicalQuantity : ValueObject { ... }
My questions:
What would be the best way to notify variants of the parameter change? I can think of two ways using events.
First would be using ArticleParameterChanged(ArticleId, parameter.Id, parameterValue.Id). I would handle this event and changed all variants at once in the handler - I don't think this is the way, but I wouldn't need to hold variants collection in article.
Second would be to loop through variant IDs and create ArticleVariantParameterChanged(ArticleId, VariantId, parameterId, parameterValueId) event. This seems more correct to me?
if (syncToVariants)
{
foreach (var variantId in _variants)
{
AddEvent(new ArticleVariantParameterChanged(Id, variantId, parameter.Id, parameterValue.Id);
}
}
How do I add new variant to article? The easiest way would be to create new variant and update the article in one transaction.
// Article method
public Variant RegisterVariant(VariantId variantId, ...)
{
var variant = new Variant(variantId, ...);
_variants.Add(variantId);
return variant;
}
// command handler? or domain service?
var article = await _articleRepo.GetAsync(articleId);
var variant = article.RegisterVariant(variantId, ...);
await _variantRepo.AddAsync(variant);
await _articleRepo.UpdateAsync(article);
Or using events?
// Article method
public Variant RegisterVariant(VariantId variantId, ...)
{
var variant = Variant.Register(variantId, this.Id, ...);
return variant;
}
// Variant static method
public Variant Register(VariantId variantId, ArticleId articleId, ...)
{
var variant = new Variant(variantId, articleId, ...);
variant.AddEvent(new VariantRegistered(variantId, articleId));
return variant;
}
// command handler
var variant = article.RegisterVariant(...);
await _variantRepo.AddAsync(variant);
// VariantRegisteredHandler
article.AddVariant(variantId);
However here it seems kind of confusing to me, article.RegisterVariant and article.AddVariant... Maybe it's just wrong naming?
Also here can occur condition race between adding new variant and assigning a new parameter, when someone adds new parameter before the VariantRegistered event was handled, so it wouldn't sync that parameter.
So I'm thinking, is it even good idea to store those shared parameters in each variant? Maybe it would be enough to just have variant specific parameters there and merge everything in the read model? However this would be harder to prevent duplications - if the article already has a parameter "color - red", assigning "color - red" to variant would need to check the article parameters too and there can be another race condition.
I read that entities without any domain business logic could be treated as CRUD, that means they wouldn't even inherit AggregateRoot and each of them would have own repository, right?
Let's say someone really wants to delete some parameter value, for example blue color. This wouldn't (hopefully) happen in my app, but I'm still curious how this would be handled. He confirms he really wants to delete it and I need to go through all articles and unassign it from them. How?
My idea would be either to have ParameterValueDeleted event and ParameterValueDeletedHandler would query for all articles and variants and unassign it one by one, this handler would take really long time to execute.
Or ParameterValueDeletedHandler would query for all IDs, create some event for them and that handler would unassign it later. However in the latter case I don't know how that event would be named to make sense. UnassignArticleParameter seems more like command than event and ArticleParameterUnassigned is something coming from article. Also I read that commands indicate something that can be rejected, so I would say command doesn't fit here.
Also I see a problem when someone deletes that parameter and someone else queries for an article which doesn't have it unassigned yet - database join would fail because it would join to non existent parameter (considering single database for read and write model).
If I wanted to have mandatory parameters, where would be the best place to validate that all of them are set? Move the article registration logic to ArticleFactory and check it there? And for variants maybe ArticleService or VariantFactory? This seems kinda inconsistent to me, but maybe it's right?
var article = await _articleRepo.GetAsync(articleId);
_articleService.RegisterVariant(article, /* variant creation data */);
_variantFactory.Register(article, /* variant creation data */);
I think this should be all, I hope I explained everything well.
I would appreciate any help with this!

I have created a form with a list inside of it, and I am trying to access that list in a new form

I am working on an assignment, where I need to have the user input the name and time of a competitor in a race. Once they input the form, they can go to a "View" screen (which is my new form) which will allow them to view the times.
Each race has 6 categories, so I created 6 lists in the main form (with the input). These lists are created based on an object called Competitor (which requires the name and time)
Now I need to take that list, and sort it in the "View" screen.
I am having trouble with this (I am very inexperienced with C#)
I tried referencing the input form, but to no avail. I know I am doing something wrong, but I don't know what it is.
public List<RaceCompute.Competitor> egg_adult_list = new List<RaceCompute.Competitor>();
public List<RaceCompute.Competitor> egg_teen_list = new List<RaceCompute.Competitor>();
public List<RaceCompute.Competitor> egg_kids_list = new List<RaceCompute.Competitor>();
public List<RaceCompute.Competitor> sack_adult_list = new List<RaceCompute.Competitor>();
public List<RaceCompute.Competitor> sack_teen_list = new List<RaceCompute.Competitor>();
public List<RaceCompute.Competitor> sack_kids_list = new List<RaceCompute.Competitor>();
public input_form()
{
InitializeComponent();
}
public class RaceCompute
{
public class Competitor
{
public string Name { get; set; }
public double Time { get; set; }
public Competitor(string name, double time)
{
Name = name;
Time = time;
}
}
}
// VIEW FORM //
input_form test2 = new input_form();
EDIT: Thanks a lot to the quick reply. I set the lists to public static and I can now access those lists. Again, I am very inexperienced but thanks for the help

The code will not look pretty at the end however you can sort the List using Where or FirstOrDefault.
egg_kids_list.FirstOrDefault(i=> i.Name = NameToSearchFor);
Either do this 6 times for all your lists and then add all the result Competitor objects into another List and pass it in to your new form as a parameter.
DisplayList.Add(egg_kids_list_item_returned_by_filter);
Or a better option might be passing your first Form as a parameter to your
second form
private class display_class
{
public display_class(data_class data)
{
//Here you can already access your lists
public List<RaceCompute.Competitor> egg_adult_list = data.egg_adult_list;
}
}

Filter Out List with another list using LINQ

Hi I'm having difficulty finding an answer to my question here, so I figured I'd just ask. I have to lists of classes, ServiceItem, and ServiceDetailsClass. I want to filter out all of the ServiceDetailClass Items that are not int ServiceItems list. Here are the two classes:
public class ServiceItem
{
public long ID { get; set; }
public string Status { get; set; }
}
public class ServiceDetailsClass
{
public string Name;
public long ID;
public int Quantity;
public string Notes;
public string Status;
public string Description;
public DateTime CreatedDate;
}
So far the only things I've found on here is for lists that have a list in them, so this is a bit different. This is all I was able to come up with, but the filter list has 0 item, even though I know it should have more than that:
lstFilteredServiceDetailsClass = lstServiceDetailsClass.Where(i => lstServiceItem.Contains
(new ServiceItem { lngId = i.ServiceID, strStatus = "Locked" })
Any help is appreciated. Thanks!

You're making a new object and then checking the list to see if that exact object/instance is in it (i.e. because it's an object, it's comparing the reference).
Instead, you need to look for overlapping IDs.
Something like this should work:
List<ServiceItem> serviceItems;
List<ServiceItemDetails> serviceItemDetails;
var result = serviceItemDetails.Where(sid => serviceItems.Any(si => si.ID == sid.ID))
In English: "The collection of ServiceItemDetails where the list of service items has an item with the same ID"

List in a List (Associations)

Trying to further my own understanding, I'm replication a simple database - and having trouble understanding the following;
I have 2 classes Town and People. A town owns many instances of People and are set like this;
public class Town
{
List<People> collectionOfPeople;
public string townName { get; set; }
public Town()
{
townName = "Cardiff";
collectionOfPeople = new List<People>();
collectionOfPeople.Add(new People("Daniel Smith"));
}
}
public class People
{
public string name { get; set; }
public People(string tmp_name)
{
name = tmp_name;
}
}
Assuming what I've done is correct, Town has 1 value (Cardiff) and People also has one (Daniel Smith) or .. Daniel lives in Cardiff.
I am trying to display the names of People living within the Town.. to later cycle through them. (** = problem I think)
private List<Town> townList;
private List<Town.People> peopleList; **
private void ShowData()
{
// Add to Text Box based on current Record
txt_town.Text = townList[0]).townName;
txt_name.Text = peopleList[0].name; **
}

Here are my changes. Provide public access modifier for collectionOfPeople in Town class.
public class Town
{
public List<People> collectionOfPeople;
public string townName { get; set; }
}
After that, you can access People instance within Town. Something like this:
private List<Town> townList = new List<Town>();
private void ShowData()
{
// Add to Text Box based on current Record
txt_town.Text = townList[0].townName;
txt_name.Text = townList[0].collectionOfPeople[0].name
}

You haven't said what, if any, error messages you're getting but I believe in order to have
Town.People
you need to create a property of your Town class called People. I don't see that in your code.
Also, there's an extra parenthesis in your line:
txt_town.Text = townList[0].townName: //no ) after [0]

C# class organization and lists

I am working on a C# application which consists of objects Department, Course, and Section. Each Department has many Courses, and each Course has many Sections. Currently I have three classes: Department, Course, and Section. Department contains some properties and then a List Courses, which contains the courses the department offers. Course contains some properties and then a List Sections, which contains the sections of the course. Is this a good way to have the code structured or should I be doing it a different way?
Secondly, when I instantiate a department in my application, I set some properties and then would like to begin adding courses to the List Courses defined in the Department class. However, I seem to be unable to simply do Department.Courses.Add(Course) from the application. What must I do within the Department class so that I may add objects to that list without breaking the principle of encapsulation?
An example of what I have with the list right now is:
class Department
{
// ......
List<Course> Courses = new List<Course>;
}
however Department.Courses is not available in the program code after the class has been instantiated (all other properties of the class are available).

Instantiate the internal Courses list inside the parameterless constructor of your class.
private List<Course> _coursesList;
public Department()
{
_coursesList = new List<Course>();
}
Also, another way to ensure the encapsulation is to provide a method on your Department class to add the courses to it instead of directly exposing the courses list. Something like
public void AddCourse(Course c) { ... }
// or (adding the feature of doing the method calls in a composable way)
public Course AddCourse(Course c) { ... }
// or
public void AddCource(String name, etc) { ... }
I think in your case it is not a good idea do directly exposes the List because the class List, may provide methods like, Add and Remove which could potentially creates an invalid state on your parent class. So if you choose to expose methods to manipulate the internal collections like I suggested, you could expose an array of Courses to your API clients (remember the arrays are read-only) so your API consumers won't be able to the create side effects on your department class.
public Course[] Courses {
get { return _coursesList.ToArray(); }
}
In addition, you could also implement the IEnumerable interface on your Department class. It would enable you to take advantage of the all LINQ extension methods available in C# 3.0.
I hope it helps,
Carlos.

Probably something Similar. There are several ways of soing this. depends upon what your requirements are.
public class Department
{
// Initialize the list inside Default Constructor
public Department()
{ courses = new List<Course>(); }
// Initialize List By Declaring outside and Passing with Dpartment Initilization
public Department(List<Course> _courses)
{ courses = _courses; }
List<Course> courses;
public List<Course> Courses
{
get
{
if (courses == null)
return new List<Course>();
else return courses;
}
set { courses = value; }
}
internal bool AddCourseToCourses(Course _course)
{
bool isAdded = false;
// DoSomeChecks here like
if (!courses.Contains(_course))
{
courses.Add(_course);
isAdded = true;
}
return isAdded;
}
}
public class Course
{
public Course(List<Subject> _subject)
{ subjects = _subject; }
List<Subject> subjects;
public List<Subject> Subjects
{
get { return subjects; }
set { subjects = value; }
}
}
// I do not get what do you mean by course "section", very general.
// used Subject instead, Change as you want just to give an idea
public class Subject
{
string name;
public string Name
{
get { return name; }
set { name = value; }
}
int creditHours;
public int CreditHours
{
get { return creditHours; }
set { creditHours = value; }
}
public Subject(string _name, int _creditHours)
{
name = _name;
creditHours = _creditHours;
}
}
public class TestClass
{
public void DoSomething()
{
// Subjects
Subject subj1 = new Subject("C#", 10);
Subject subj2 = new Subject(".Net", 10);
// List of Subjects
List<Subject> advancedPrSubjects = new List<Subject>();
advancedPrSubjects.Add(subj1);
advancedPrSubjects.Add(subj2);
// Course
Course advancedProgramming = new Course(advancedPrSubjects);
// Deliver authoroty to add Course to Department Class itself
Department dept = new Department();
dept.AddCourseToCourses(advancedProgramming);
}
}
There are better ways of doing this. have a look at these tutorials for better insight
http://www.csharp-station.com/Tutorials/Lesson07.aspx
http://www.functionx.com/csharp/index.htm
Hope it helps

As to your second question - without some code or more details its a bit hard - but i'll take a guess.
You're probably not actually creating the list, just declaring it
List<xxxx> _variable;
vs
List<xxxx> _variable = new List<xxxxx>();
You must create a list to be able to add to it (new List());

You sound as if you're on the right track.
Your second problem could be down to many things.
It could be as Ruddy says and that you're not creating the list.
It could also be that your Courses List is not public or that you haven't instanciated a new Course object to add.