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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).

Derived derived;
Base base = derived;

Covariance

Allow a generic object with derived type to be assigned to a generic object of base type.

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

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> { }

Contravariance

Allow a generic object with base type to be used for a generic object of derived type.

// an equality comparer for Base type, used with Derived objects
class BaseComparer : IEqualityComparer<Base>
{
    public bool Equals(Base? x, Base? y) => x?.Id == y?.Id;
    public int GetHashCode(Base obj) => obj.Id;
}

var derived1 = new Derived { Id = 1 };
var derived1bis = new Derived { Id = 1 };
var derived2 = new Derived { Id = 2 };

var baseComparer = new BaseComparer();
var b3 = baseComparer.Equals(derived1, derived1bis);
var b4 = baseComparer.Equals(derived1, derived2);

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