public void Covariance()
{
ICovariant <Fruit> fruit = new Covariant <Fruit>();
ICovariant <Apple> apple = new Covariant <Apple>();
Covariant(fruit);
Covariant(apple); //apple is being upcasted to fruit, without the out keyword this will not compile
}
static void ContravariantAndCovariant()
{
IContravariant <BaseClass> iBaseClassA = new Contravariant <BaseClass>();
iBaseClassA.Method(new DerivedClass());
ICovariant <BaseClass> iBaseClassB = new Covariant <BaseClass>();
iBaseClassB.Method().Print();
}
private static void Covariance()
{
ICovariant <Fruit> fruit = new Covariant <Fruit>();
ICovariant <Fruit> apple = new Covariant <Apple>();
// ICovariant<Apple> fruit = new Covariant<Fruit>(); // not allowed
// Covariant<Fruit> appleNotCompiled = new Covariant<Apple>(); // won't compile
// Implementations are always invariant, use interfaces to achieve variance
Console.WriteLine(fruit.GetType());
Console.WriteLine(apple.GetType());
}
private void CovarianceExample()
{
ConsoleExtensions
.WriteColoredLine(Resources.CovarianceExampleRunning, ConsoleColor.Cyan);
// A covariant type in an interface or delegate can be converted to a more GENERAL type.
// Interface: ICovariant<Banana> to ICovariant<Fruit>
ICovariant <Fruit> fruitCovariantInterface = this.bananaCovariantInterface;
// Delegate: Covariant<Lemon> to Covariant<object>
Covariant <object> objectCovariantDelegate = this.lemonCovariantDelegate;
const int FruitQuality = 85;
var fruit = fruitCovariantInterface.Create(FruitQuality);
var obj = objectCovariantDelegate();
Console.WriteLine(Resources.CovarianceExampleFruitColor, fruit.Name, fruit.Color);
Console.WriteLine(Resources.CovarianceExampleObjString, nameof(obj), obj.ToString());
Console.WriteLine();
}
static void Run()
{
ICovariant <Fruit> apple = new Covariant <Apple>(); //because it's covariant
IContravariant <Apple> fruit = new Contravariant <Fruit>(); //because it's contravariant
}
public void Example()
{
// possible because its covariant (out keyword)
ICovariant <Fruit> fruit = new Covariant <Apple>();
}
