« 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) type or a more-derived type (contravariance).<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.

Version du 26 mars 2024 à 16:49

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Description

Variance allows to replace a type with a less-derived (covariance) type or a more-derived type (contravariance).
Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.
Covariance preserves assignment compatibility and contravariance reverses it.

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 strings = new List<string>();
List<object> objects = strings;   // Cannot convert type List<string> to List<object> because there is no covariance with List<T>
IList<object> objects = strings;  // Cannot implicitly convert type List<string> to IList<object>. An explicit conversion exists

IEnumerable<object> objects = strings; // There is covariance with IEnumerable<T>

var objects = (IList<object>)strings; // There is covariance with IList<T>

Native covariant interfaces

Interface Since
IEnumerable<out T> .NET Framework 4.0
IReadOnlyList<out T> .NET Framework 4.5
IReadOnlyCollection<out T> .NET Framework 4.5