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();
}
}
}
