Variance
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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).
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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).
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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 |
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> { }
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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 |
