« 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 enable implicit reference conversion for array types, delegate types, and generic type arguments.<br>Covariance preserves assignment compatibility and contravariance reverses it. | ||
Covariance 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.
Covariance preserves assignment compatibility and contravariance reverses it. - Contravariance
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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).
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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).
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It works only with covariant interface |
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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 |
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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> { }
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Native variant interfaces
| Interface | Since |
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| IEnumerable<out T> | .NET Framework 4.0 |
| IReadOnlyList<out T> | .NET Framework 4.5 |
| IReadOnlyCollection<out T> | .NET Framework 4.5 |
