Array covariance is the principle that if an implicit or explicit reference conversion exits from type ``++A++`` to ``++B++``, then the same conversion exists from the array type ``++A[]++`` to ``++B[]++``.


While this array conversion can be useful in readonly situations to pass instances of ``++A[]++`` where ``++B[]++`` is expected, it must be used with care, since assigning an instance of ``++B++`` into an array of ``++A++`` will cause an ``++ArrayTypeMismatchException++`` to be thrown at runtime.


== Noncompliant Code Example

----
abstract class Fruit { }
class Apple : Fruit { }
class Orange : Fruit { }

class Program
{
  static void Main(string[] args)
  {
    Fruit[] fruits = new Apple[1]; // Noncompliant - array covariance is used
    FillWithOranges(fruits);
  }

  // Just looking at the code doesn't reveal anything suspicious
  static void FillWithOranges(Fruit[] fruits)
  {
    for (int i = 0; i < fruits.Length; i++)
    {
      fruits[i] = new Orange(); // Will throw an ArrayTypeMismatchException
    }
  }
}
----


== Compliant Solution

----
abstract class Fruit { }
class Apple : Fruit { }
class Orange : Fruit { }

class Program
{
  static void Main(string[] args)
  {
    Orange[] fruits = new Orange[1]; // Compliant
    FillWithOranges(fruits);
  }

  static void FillWithOranges(Orange[] fruits)
  {
    for (int i = 0; i < fruits.Length; i++)
    {
      fruits[i] = new Orange();
    }
  }
}
----