PInvoke return struct with two dimensions array

Question

I have struct which is defined in c++ Win32 DLL like the following:

typedef struct matrix
{
  double** data;
  int m;
  int n;
} Matrix;

And there is a function:

Matrix getMatrix(void);

Matrix getMatrix()
{
    Matrix mat;

    mat.m = 2;
    mat.n = 2;

    mat.data    = (double**)  malloc (sizeof(double*) * 4);

    mat.data[0] = (double* )  malloc (sizeof(double) * 2);
    mat.data[1] = (double* )  malloc (sizeof(double) * 2);

    mat.data [0][0]=1;
    mat.data [0][1]=2;
    mat.data [1][0]=3;
    mat.data [1][1]=4;

    return mat;
}

How can I capture the return value of this function If I'm using P/Invoke from a C# Application

I would think double** translates to double[][]. Not sure if you have to decorate it with an attribute though to get it to work right. — lc.
– lc., Commented Sep 24, 2012 at 8:37
You declare data to be double** but actually it is int**. Fix that and the technique of @user629926's answer will get you home. — David Heffernan
– David Heffernan, Commented Sep 24, 2012 at 12:37
You first malloc is wrong. It should be mat.data = (int**) malloc (sizeof(int*) * mat.m) but better would be (int**) malloc(sizeof(*mat.data)*mat.m) — David Heffernan
– David Heffernan, Commented Sep 24, 2012 at 12:39

Lorenz Lo Sauer · Accepted Answer · 2012-09-24 13:06:20Z

5

I am not sure if it works, but from memory: Declare data as IntPtr and use this :

static double[][] FromNative (IntPtr data, int m,int n)
{
   var matrix=new double[m][];

   for(int i=0;i<m;i++)
   {
       matrix[i]=new double[n];
       Marshal.Copy(Marshal.ReadIntPtr(data),matrix[i],0,n);
       data =(IntPtr)(data.ToInt64()+IntPtr.Size);
   }

   return matrix;
}

edited Sep 24, 2012 at 13:06

Lorenz Lo Sauer

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answered Sep 24, 2012 at 9:19

user629926

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14 Comments

Viktor Latypov Over a year ago

The 'new double[]' would be in managed memory. And double arrays in C# are objects with different layout. Unfortunately, your approach won't work.

user629926 Over a year ago

Sorry if someone can format my answer,my mobile editor wont format it right. If he wants double[,] instead of double[][] I can edit the question?

Viktor Latypov Over a year ago

It's better, but the problem is still with the 2D-ness of the array. The zig-zag array you propose won't map the '**' pointer.

user629926 Over a year ago

I just asumed that by double** he wanted array of double arrays.

user629926 Over a year ago

What do you mean by free. If you wish to free memory alocated by struct from c# . You should make your own free routine ,export it and call lt from c#.

|

Community · Accepted Answer · 2017-05-23 11:52:16Z

1

The answer is short but disappointing: you have to convert the multidimensional array to the single-dimensional one.

There is an answer using AllocHGlobal: Pass multi - dimensional array from managed code to unmanaged code

This solution does similar thing, but in the C# definition in your case you have to make the data field to be of IntPtr type and in unmanaged code you have to assume it is a single-dimensional array.

There is another solution using Marshal.UnsafeAddrOfPinnedArrayElement (MSDN and StackOverflow), but it still assumes you are using the one-dimensional arrays.

Better options would follow if you tell what is the problem you are trying to solve. Is it a pure return-from-one-very-important-function-and-forget-this-PInvoke or are you trying to do a constant bidirectional marshalling ?

edited May 23, 2017 at 11:52

CommunityBot

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answered Sep 24, 2012 at 8:54

Viktor Latypov

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1 Comment

David Heffernan Over a year ago

You certainly do not need to convert to single dimensional

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