« Variance » : différence entre les versions
De Banane Atomic
Aller à la navigationAller à la recherche
| Ligne 42 : | Ligne 42 : | ||
interface IVariant<out T> { } | interface IVariant<out T> { } | ||
class Variant<T> : IVariant<T> { } | class Variant<T> : IVariant<T> { } | ||
</kode> | |||
= Contravariance = | |||
<kode lang='cs'> | |||
class BaseComparer : IEqualityComparer<Base> | |||
{ | |||
public bool Equals(Base? x, Base? y) => x?.Id == y?.Id; | |||
public int GetHashCode([DisallowNull] Base obj) => obj.Id; | |||
} | |||
var derived1 = new Derived { Id = 1 }; | |||
var derived1bis = new Derived { Id = 1 }; | |||
var derived2 = new Derived { Id = 2 }; | |||
var baseComparer = new BaseComparer(); | |||
var b3 = baseComparer.Equals(derived1, derived1bis); | |||
var b4 = baseComparer.Equals(derived1, derived2); | |||
</kode> | </kode> | ||
Version du 27 mars 2024 à 16:11
Links
Description
Variance allows to replace a type with a less-derived (covariance: derived → base) type or a more-derived type (contravariance: base → derived).
It is available for array types, delegate types, and generic types.
Assignment compatibility
Allow an object of a more derived type (derived) to be assigned to an object of a less derived type (base).
|
Derived derived; Base base = derived; |
Covariance
Allow a generic object of child class type (ex: string) to be assigned to a generic object of parent class type (ex: object).
 |
It works only with covariant interface |
|
|
var deriveds = new List<Derived>(); List<Base> bases = strings; // Cannot convert type List<Base> to List<Derived> because List<T> is invariant IList<Base> bases = strings; // Cannot implicitly convert type List<Base> to IList<Derived> because IList<T> is invariant var bases = (IList<Base>)deriveds; // at execution, Unable to cast List<Derived> to type IList<Base> IEnumerable<Base> bases = deriveds; // IEnumerable<T> is covariant IReadOnlyList<Base> bases = deriveds; // IReadOnlyList<T> is covariant IReadOnlyCollection<Base> bases = deriveds; // IReadOnlyCollection<T> is covariant |
Create a covariant interface
|
|
var derivedVariants = new VariantList<Derived> { new Derived() };
IVariantList<Base> baseVariants = derivedVariants;
var derivedVariant = new Variant<Derived>();
IVariant<Base> baseVariant = derivedVariant;
interface IVariantList<out T> { }
class VariantList<T> : List<T>, IVariantList<T> { }
interface IVariant<out T> { }
class Variant<T> : IVariant<T> { }
|
Contravariance
|
|
class BaseComparer : IEqualityComparer<Base>
{
public bool Equals(Base? x, Base? y) => x?.Id == y?.Id;
public int GetHashCode([DisallowNull] Base obj) => obj.Id;
}
var derived1 = new Derived { Id = 1 };
var derived1bis = new Derived { Id = 1 };
var derived2 = new Derived { Id = 2 };
var baseComparer = new BaseComparer();
var b3 = baseComparer.Equals(derived1, derived1bis);
var b4 = baseComparer.Equals(derived1, derived2);
|
Native variant interfaces
| Interface | Since |
|---|---|
| IEnumerable<out T> | .NET Framework 4.0 |
| IReadOnlyList<out T> | .NET Framework 4.5 |
| IReadOnlyCollection<out T> | .NET Framework 4.5 |
