Returning a variable sized array of doubles from C++ to C# - a simpler way?

Question

I have the following C++ method :

__declspec(dllexport) void __stdcall getDoubles(int *count, double **values); the method allocates and fills an array of double and sets *count to the size of the array.

The only way i managed to get this to work via pinvoke is :

[System.Runtime.InteropServices.DllImportAttribute("xx.dll")]
 public static extern void getDoubles(ref int count, ref System.IntPtr values);

and usage is :

int count = 0;
IntPtr doubles = new IntPtr();
Nappy.getDoubles(ref count, ref doubles);
double[] dvs = new double[count];
for(int i = 0;i < count;++{
    dvs[i] = (double)Marshal.PtrToStructure(doubles, typeof(System.Double));
    doubles = new IntPtr(doubles.ToInt64()+Marshal.SizeOf(typeof(System.Double)));
}

the values end up in the dvs array. Is there a better way ti do this not invloving pointer arithmetic in a managed language...

The description is incomplete - HOW does it allocate the memory, and how is it returned to that allocator? — MSalters
– MSalters, Commented Jan 14, 2009 at 13:01

Ahmed · Accepted Answer · 2009-01-14 12:32:38Z

3

I think you can use

Marshal.Copy( source, destination, 0, size );

answered Jan 14, 2009 at 12:32

Ahmed

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Comments

arul · Accepted Answer · 2009-01-14 12:56:21Z

1

You'll need to change the unmanaged method signature so it reads like this:

__declspec(dllexport) void __stdcall getDoubles(SAFEARRAY *array);

Then you should be able to use the following managed version of the function:

[System.Runtime.InteropServices.DllImportAttribute("xx.dll")]
public static extern void getDoubles(
              [MarshalAs(UnmanagedType.SafeArray, SafeArraySubType=VT_R8)]
              double[] array
);

Of course that you'll also have to rewrite your unmanaged code to work with SAFEARRAYs. More about this topic can be found at MSDN.

One question though, I recall working with ZLib in C# which is able, without any wrapper, to work with byte[] while the unmanaged counterpart is BYTE*, have you tried working directly with double* / double[] ?

answered Jan 14, 2009 at 12:56

arul

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1 Comment

zohar Over a year ago

I have tried using double [] or ref double directly but that only work for a single level of indirection. there is support for marshalling an array of doubles if you know the size on the managed side.

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