« Variance » : différence entre les versions

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= [https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance Description] =
= [https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance Description] =
Variance allows to replace a type with a less-derived (covariance: derived → base) type or a more-derived type (contravariance: base → derived).<br>
Variance allows to replace a type with a less-derived (covariance: derived → base) type or a more-derived type (contravariance: base → derived).<br>
* Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.<br>Covariance preserves assignment compatibility and contravariance reverses it.
* Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.<br>It preserves assignment compatibility and contravariance reverses it.
* Contravariance
* Contravariance
{{info | Since .NET Framework 4, C# supports covariance and contravariance in generic interfaces and delegates and allows for implicit conversion of generic type parameters.}}
{{info | Since .NET Framework 4, C# supports covariance and contravariance in generic interfaces and delegates and allows for implicit conversion of generic type parameters.}}

Version du 27 mars 2024 à 15:50

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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).

  • Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.
    It preserves assignment compatibility and contravariance reverses it.
  • Contravariance
Since .NET Framework 4, C# supports covariance and contravariance in generic interfaces and delegates and allows for implicit conversion of generic type parameters.

Assignment compatibility

Allow an object of a more derived type (child class, ex: string) to be assigned to an object of a less derived type (parent class, ex: object).

Cs.svg
string s = "test";
object o = s;

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
Cs.svg
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
Cs.svg
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> { }

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