C++/CLI managed pointer function param being used in C#

Question

I have this code wrapper, being build as a dll.

namespace VoxelEditor
{
  using namespace System;
  public ref class name
  {

  private: VoxelEngine::test::name* _name;

  public: 
    name()
    {
      _name = new VoxelEngine::test::name; 
    }

    ~name()
    { 
      delete _name;
    }

  void test(IntPtr result, int a, int b)
  { 
    _name->test((int*)result.ToPointer(), a, b); 
  }

  };
}

namespace VoxelEngine
{
  namespace test
  {


    void name::test(int*result, int a, int b)
    {
      *result = a + b;
    }

  }
}

I want to create this name class in c# and call the function test. I read it somewhere I should use IntPtr

int d = 0; 
test.test(d, i, b);
Console.WriteLine(d);

question 1: It crashes, said that the "An exception of type 'SYstem.NullReferenceExeption' occured

question 2: What if a function returns a pointer? I have read this: https://msdn.microsoft.com/en-us/library/367eeye0.aspx And found some answers. But I don't get it. I am new on this.

You should pass d as ref.

cbr
– cbr

2015-04-26 14:38:31 +00:00
Commented Apr 26, 2015 at 14:38 — cbr
– cbr, Commented Apr 26, 2015 at 14:38

Hans Passant · Accepted Answer · 2015-04-26 14:45:45Z

Pass the argument by reference instead so the C# code can simply use ref d in the method call. A stable copy of it is required so you can pass a pointer to the native code. Like this:

  void test(int% result, int a, int b) { 
      int copy = result;
      _name->test(&copy, a, b); 
      result = copy;
  }

Also note that you must implement the finalizer so you don't leak memory when the C# programmer forgets to dispose the object. It also never hurts to ensure that nothing goes wrong when the C# program disposes more than once:

    ~name() {
        if (_name != nullptr) {
            this->!name();
            _name = nullptr;
        }
    }
    !name() { 
      delete _name;
    }

Your 2nd question is very broad, you'd generally pass such a pointer to the constructor of the ref class that wraps the underlying object. And return a reference to it.

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