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How to: Marshal Embedded Pointers Using C++ Interop

The following code examples use the managed, unmanaged #pragma directives to implement managed and unmanaged functions in the same file, but these functions interoperate in the same manner if defined in separate files. Files containing only unmanaged functions do not need to be compiled with /clr (Common Language Runtime Compilation).

The following example demonstrates how an unmanaged function that takes a structure containing pointers can be called from a managed function. The managed function creates an instance of the structure and initializes the embedded pointer with the new keyword (instead of the gcnew keyword). Because this allocates the memory on the native heap, there is no need to pin the array to suppress garbage collection. However, the memory must be explicitly deleted to avoid memory leakage.

// marshal_embedded_pointer.cpp
// compile with: /clr
#include <iostream>

using namespace System;
using namespace System::Runtime::InteropServices;

// unmanaged struct
struct ListStruct {
   int count;
   double* item;
};

#pragma unmanaged

void UnmanagedTakesListStruct(ListStruct list) {
   printf_s("[unmanaged] count = %d\n", list.count);
   for (int i=0; i<list.count; i++)
      printf_s("array[%d] = %f\n", i, list.item[i]);
}

#pragma managed

int main() {
   ListStruct list;
   list.count = 10;
   list.item = new double[list.count];

   Console::WriteLine("[managed] count = {0}", list.count);
   Random^ r = gcnew Random(0);
   for (int i=0; i<list.count; i++) {
      list.item[i] = r->NextDouble() * 100.0;
      Console::WriteLine("array[{0}] = {1}", i, list.item[i]);
   }

   UnmanagedTakesListStruct( list );
   delete list.item;
}
[managed] count = 10 array[0] = 72.624326996796 array[1] = 81.7325359590969 array[2] = 76.8022689394663 array[3] = 55.8161191436537 array[4] = 20.6033154021033 array[5] = 55.8884794618415 array[6] = 90.6027066011926 array[7] = 44.2177873310716 array[8] = 97.754975314138 array[9] = 27.370445768987 [unmanaged] count = 10 array[0] = 72.624327 array[1] = 81.732536 array[2] = 76.802269 array[3] = 55.816119 array[4] = 20.603315 array[5] = 55.888479 array[6] = 90.602707 array[7] = 44.217787 array[8] = 97.754975 array[9] = 27.370446

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