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).
Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.
Covariance preserves assignment compatibility and contravariance reverses it.
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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<string> to List<object> because List<T> is invariant IList<Base> bases = strings; // Cannot implicitly convert type List<string> to IList<object> because IList<T> is invariant IEnumerable<Base> bases = deriveds; // IEnumerable<T> is covariant IReadOnlyList<Base> bases = deriveds; // IReadOnlyList<T> is covariant IReadOnlyCollection<Base> bases = deriveds; // IReadOnlyCollection<T> is covariant var bases = (IList<Base>)deriveds; // ??? |
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 |
