NGWS SDK Documentation  

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11.2 Array covariance

For any two reference types A and B, if A is a derived type of B, then a conversion exists from an array of type A to a compatible (that is, rank and type of array) array of type B. This relationship is known as array covariance. Array covariance in particular means that an element of an array whose element type is B may actually be an element of an array whose element type is A, provided both A and B are reference types and B is a base type of A.

Because of array covariance, assignments to elements of reference type arrays include a runtime check that ensures that the value being assigned to the array element is actually of a permitted type. Array covariance specifically does not extend to arrays of value types.