Marshal C++ int array to C#

Question

I would like to marshal an array of ints from C++ to C#. I have an unmanaged C++ dll which contains:

DLL_EXPORT int* fnwrapper_intarr()
{
    int* test = new int[3];

    test[0] = 1;
    test[1] = 2;
    test[2] = 3;

    return test;
}

with declaration in header extern "C" DLL_EXPORT int* fnwrapper_intarr();

I am then using pinvoke to marshal it into C#:

[DllImport("wrapper_demo_d.dll")]
[return: MarshalAs(UnmanagedType.SafeArray)]
public static extern int[] fnwrapper_intarr();

And I use the function like so:

int[] test = fnwrapper_intarr();

However, during program execution I get the following error: SafeArray cannot be marshaled to this array type because it has either nonzero lower bounds or more than one dimension.

What array type should I be using? Or is there a bettery way of marshalling arrays or vectors of integers?

If you are the author of fnwrapper_intarr, is there anything against returning a safearray (or better as parameter)? — Liton
– Liton, Commented Sep 23, 2010 at 9:01

Alex F · Accepted Answer · 2014-08-15 19:25:42Z

21

[DllImport("wrapper_demo_d.dll")]
public static extern IntPtr fnwrapper_intarr();

IntPtr ptr = fnwrapper_intarr();
int[] result = new int[3];
Marshal.Copy(ptr, result, 0, 3);

You need also to write Release function in unmanaged Dll, which deletes pointer created by fnwrapper_intarr. This function must accept IntPtr as parameter.

DLL_EXPORT void fnwrapper_release(int* pArray)
{
    delete[] pArray;
}

[DllImport("wrapper_demo_d.dll")]
public static extern void fnwrapper_release(IntPtr ptr);

IntPtr ptr = fnwrapper_intarr();
...
fnwrapper_release(ptr);

edited Aug 15, 2014 at 19:25

answered Sep 23, 2010 at 8:41

Alex F

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2 Comments

Seth Over a year ago

+1 Great! It worked, except I think it should be Marshal.Copy not Marshal::Copy.

Gleno Over a year ago

And what should you do if you don't know the size of the array on C# side?

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