Variance

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• Generics

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.

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).

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

var strings = new List<string>(); List<object> objects = strings; // Cannot convert type List<string> to List<object> because List<T> is invariant IList<object> objects = strings; // Cannot implicitly convert type List<string> to IList<object> because IList<T> is invariant IEnumerable<object> objects = strings; // IEnumerable<T> is covariant var objects = (IList<object>)strings; // ???

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