Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
Export (0) Print
Expand All

4.5.6 Array Covariance

Visual Studio .NET 2003

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


// __gc_arrays8.cpp
// compile with: /clr
#using <mscorlib.dll>
using namespace System;

int main() {
   Object* oa[] = new String*[20];  // String derives from Object


  • 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.


    // __gc_arrays9.cpp
    // compile with: /clr
    #using <mscorlib.dll>
    using namespace System;
    __gc struct Base { int i; };
    __gc struct Derived  : Base {};
    __gc struct Derived2 : Base {};
    __gc struct Derived3 : Derived {};
    __gc struct Other { short s; };
    int main() {
       Derived* d[] = new Derived*[100];
       // ok by array covariance
       Base*   b[] = d;
       // invalid
       // b[0] = new Other;
       // error (runtime exception)
       // b[1] = new Derived2;
       // error (runtime exception),
       // must be "at least" a Derived.
       // b[0] = new Base;      
       b[1] = new Derived;
       b[0] = new Derived3;

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

© 2015 Microsoft