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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.
It preserves assignment compatibility and contravariance reverses it.
Contravariance

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).

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

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> { }

Native variant interfaces

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