passing 2D array from managed C++ to unmanaged C++

Question

I'm working on a managed C++ wrapper for unmanaged C++ code and have a question. Just for simplicity let's say that I need to pass a 2D array from C# code to Managed C++ to Unmanaged C++. I have no problem with 1D array but stuck with 2D version. Converting it to 1D is the option, but I want to take a look if there are other ways.

For simplicity let say I want to send 2D array to unmanaged code using intermediate wrapper and change its values.

so here is C# code with a call to managed C++:

MNumeric wrapper = new MNumeric();  //managed C++ object
int[,] dArr = new int[10, 10];
for (int i = 0; i < 10; i++)
        {
            for (int j = 0; j < 10; j++)
            {
                dArr[i, j] = 10;
            }
        }
wrapper.ChangeArray(dArr, Convert.ToInt32(Math.Sqrt(dArr.Length)))

Managed C++:

void MNumeric::ChangeArray(cli::array<int,2> ^arr, int mySize)
{
      //some code to call Unmanaged C++ passing managed 2D array ???
}

Unmanaged C++

void UNumeric::ChangeArray(int** arr, int mySize)
{
    for(int i=0;i<mySize;i++)
    {
        for(int j=0;j<mySize;j++)
        {
            arr[i][j]=i;
        }
    }
}

Thanks a lot for your help.

Looks like I fix one error but got another. My C++ Managed code looks like this now.

void MNumeric::ChangeArray(cli::array<int,2> ^arr, int mySize)
{
    pin_ptr<int> p_arr = &arr[0,0];
    u_num->ChangeArray((int**)p_arr, mySize);           
}

where u_num is just a pointer to UNumeric class. The error I got now is the following:

Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

Any ideas appreciated.

What is your question? How can you do this? Is this possible? All of the above? — RLH, Jun 15 '11 at 15:43

Alex F · Answer 1 · 2011-06-15T18:58:56.590

3

void MNumeric::ChangeArray(cli::array&ltint,2> ^arr, int mySize) 
{
    pin_ptr&ltint> p = &arr[0,0];   // pin pointer to first element in arr
    int* np = p;   // pointer to the first element in arr
    UNumeric::ChangeArray((int**)np, mySize);
}

edited Jun 15 '11 at 18:58

answered Jun 15 '11 at 15:50

Alex F

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Alex, thanks for reply, but this solution does not work. pin_ptr p = &arr[0][0] line throws error C3262: invalid array indexing: 1 dimension(s) specified for 2-dimensional 'cli::array ^' – ilyaw77 Jun 15 '11 at 16:07
Alex, I was able to fix this problem using pin_ptr p_arr = &arr[0,0]; because it is 2d array not jagged array. Code compiles fine now but when I'm trying to run I get AccessViolationException on u_num->ChangeArray((int**)np, mySize) line. – ilyaw77 Jun 15 '11 at 16:45
@lyaw77: Replace arr->Length with mySize in my answer. – Alex F Jun 15 '11 at 17:29

score 2 · Answer 2 · answered Jun 15 '11 at 20:10

2

You should not use a cast here, it could hide potentially important warnings. Notably, a 2D array is not convertible to an int**. An int**, is a pointer to an array of pointers. An int*[] is a pointer to an array of arrays. You have a function that takes an array of pointers. It wants a managed int[][], not an int[,]. Your compiler would thoroughly have shouted at you for attempting this, if you hadn't casted.

answered Jun 15 '11 at 20:10

Puppy

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Thank you for the reply. I already tried to go this way but with the same error: in C# I had: int[][] dArr = new int[10][]; in Managed C++: void MNumeric::ChangeArray(cli::array^>^ arr, int mySize){pin_ptr p_arr = &arr[0][0];u_num->ChangeArray((int**)p_arr, mySize);}, and I still have the same error. – ilyaw77 Jun 15 '11 at 20:30
@ilyaw77: That's because you never allocated the second lot of arrays. You allocated an array of pointers and then never set those pointers to point anywhere. Oh, and *take out the cast*. – Puppy Jun 16 '11 at 10:05
Thank you for advice, I was able to do it using GCHandle class. – ilyaw77 Jun 16 '11 at 19:33

passing 2D array from managed C++ to unmanaged C++

2 Answers2