This library helps in implementing Value Object classes in the context of Domain-Driven Design.
A Value Object has several traits, some of which this library provides.
A Value Object
is immutable. Every property must be read-only (i.e. no setter allowed) after instantiation.
contains domain logic and behaviours. It should encapsulate the domain complexity within it.
uses the Ubiquitous Language of the domain. A Value Object is an elegant way of embracing the language of the domain in the codebase.
exposes, uses and combines functions to provide domain value. Functions usually return new instances of a Value Object. Closure of Operations describes an operation whose return type is the same as the type of it's arguments.
// The following function doesn't change any given Amount instance, it just returns a new one.
public Amount Add(Amount amount)
{
if(this.Currency != amount.Currency)
{
throw new InvalidOperationException("Cannot add amounts with different currencies.");
}
Amount result = new Amount(this.Quantity + amount.Quantity, this.Currency);
return result;
}
uses all of it's attibutes for Equality and Uniqueness.
is upon instantiation using and throws exception if a validation fails.
automatically validated
domain validation
Usage
ValueObject<TValueObject>
By having your Value Object derive from the ValueObject<TValueObject> base class it properly implements Equality
(Equals()) and Uniqueness (GetHashCode()). Automatically all public properties are used for the calculations
without you having to write a single line of code.
This default implementation uses reflection to aquire the metadata and to get the values to use. You can provide your own
implementation simply by overriding the GetEqualityComponents() method.
A simple implementation would look like the this:
public class Amount : ValueObject<Amount>
{
public Amount(decimal quantity, Currency currency)
{
this.Quantity = quantity;
this.Currency = currency;
}
public decimal Quantity { get; }
public Currency Currency { get; }
public Amount Add(Amount amount)
{
if(this.Currency != amount.Currency)
{
throw new InvalidOperationException("Cannot add amounts with different currencies.");
}
Amount result = new Amount(this.Quantity + amount.Quantity, this.Currency);
return result;
}
}
If you decide not to use the relection-based approach, you can simply override GetEqualityComponents() and
return the values manually. Keep in mind, that all attibutes should be used for Equality and Uniqueness.
A specialized Value Object that only holds a single primitive, string or enum value.
Collections
There are implementations of IList<T>, ISet<T> and IDictionary<TKey, TValue> that determine
equality based on content and not on the collection reference.
When your Value Object contains a collection, this collection needs to be wrapped in one of the
available value collection to support the correct way for equality.
If you use the default behavior you can just wrap the collection in the constructor like below.
The default equality behavior will automatically pick the value up.
public class Confederation : ValueObject<Confederation>
{
public Confederation(string name, IList<Country> memberCountries)
{
Guard.Against.NullOrWhiteSpace(name, nameof(name));
Guard.Against.NullOrEmpty(memberCountries, nameof(memberCountries));
this.Name = name;
this.MemberCountries = memberCountries.AsValueList(); // Wrap the list in a value list.
}
public string Name { get; }
public IList<Country> MemberCountries { get; }
}
If you prefer to override GetEqualityComponents() and return the values manually you can wrap the list later.
protected override IEnumerable<object> GetEqualityComponents()
{
yield return this.Name;
yield return this.MemberCountries.AsValueList(); // Wrap the list in a value list.
}
Important Collections must be wrapped in value collections to ensure the correct equality behavior.
ValueList<T>
A list with equality based on the content instead on the list's reference, i.e. two different list instances
containing the same items in the same order will be equal.
// Wrap an IList in a ValueList.
IList<Country> countries = new List<Country>
{
Country.Create("DE"),
Country.Create("US"),
};
IList<Country> valueList = countries.AsValueList();
ValueSet<T>
A set with equality based on the content instead on the set's reference, i.e two different set instances containing
the same items will be equal regardless of their order.
// Wrap an ISet in a ValueSet.
ISet<Country> countries = new HashSet<Country>
{
Country.Create("DE"),
Country.Create("US"),
};
ISet<Country> valueSet = countries.AsValueSet();
ValueDictionary<TKey, TValue>
A dictionary with equality based on the content instead on the dictionary's reference, i.e. two different dictionary
instances containing the same items will be equal.
// Wrap an IDictionary in a ValueDictionary.
IDictionary<int, Country> countries = new Dictionary<int, Country>
{
{ 1, Country.Create("DE") },
{ 4, Country.Create("US") },
};
IDictionary<int, Country> valueDictionary = countries.AsValueDictionary();