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Array Covariance


Given reference class D with direct or indirect base class B, an array of type D can be assigned to an array variable of type B.

// clr_array_covariance.cpp
// compile with: /clr
using namespace System;

int main() {
   // String derives from Object
   array<Object^>^ oa = gcnew array<String^>(20);

An assignment to an array element shall be assignment-compatible with the dynamic type of the array. An assignment to an array element with incompatible type will cause System::ArrayTypeMismatchException to be thrown.

Array covariance does not apply to arrays of value class type. For example, arrays of Int32 cannot be converted to Object^ arrays, even via boxing.


// clr_array_covariance2.cpp
// compile with: /clr
using namespace System;

ref struct Base { int i; };
ref struct Derived  : Base {};
ref struct Derived2 : Base {};
ref struct Derived3 : Derived {};
ref struct Other { short s; };

int main() {
   // Derived* d[] = new Derived*[100];
   array<Derived^> ^ d = gcnew array<Derived^>(100);

   // ok by array covariance
   array<Base ^> ^  b = d;

   // invalid
   // b[0] = new Other;

   // error (runtime exception)
   // b[1] = gcnew Derived2;

   // error (runtime exception),
   // must be "at least" a Derived.
   // b[0] = gcnew Base;

   b[1] = gcnew Derived;
   b[0] = gcnew Derived3;
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