« Variance » : différence entre les versions
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| (2 versions intermédiaires par le même utilisateur non affichées) | |||
| Ligne 16 : | Ligne 16 : | ||
= Covariance = | = Covariance = | ||
Allow a generic object | Allow a generic object with derived type to be assigned to a generic object of base type. | ||
{{info | It works only with covariant interface}} | {{info | It works only with covariant interface}} | ||
<kode lang='cs'> | <kode lang='cs'> | ||
| Ligne 45 : | Ligne 45 : | ||
= Contravariance = | = Contravariance = | ||
Allow a generic object with base type to be used for a generic object of derived type. | |||
<kode lang='cs'> | <kode lang='cs'> | ||
// an equality comparer for Base type, used with Derived objects | |||
class BaseComparer : IEqualityComparer<Base> | class BaseComparer : IEqualityComparer<Base> | ||
{ | { | ||
public bool Equals(Base? x, Base? y) => x?.Id == y?.Id; | public bool Equals(Base? x, Base? y) => x?.Id == y?.Id; | ||
public int GetHashCode( | public int GetHashCode(Base obj) => obj.Id; | ||
} | } | ||
Dernière version du 27 mars 2024 à 16:15
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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 with derived type to be assigned to a generic object of base type.
 |
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
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|
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
Allow a generic object with base type to be used for a generic object of derived type.
|
|
// an equality comparer for Base type, used with Derived objects
class BaseComparer : IEqualityComparer<Base>
{
public bool Equals(Base? x, Base? y) => x?.Id == y?.Id;
public int GetHashCode(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 |
