Return SAFEARRAY from c++ to c#

Question

I have a c++ method which creates, fills and returns SAFEARRAY:

SAFEARRAY* TestClass::GetResult(long& size) 
{
    return GetSafeArrayList(size);
}

How should I export that function in a DLL so that c# could take it
How should I write c# method signature?

I have in c++ something along these lines:

extern "C" __declspec(dllexport) void GetResult(SAFEARRAY*& data, long& size)
{
    size = 0;
    data = handle->GetResult(size);
}

Is it correct, isn't it?

Thanks for help!

EDIT:

c# call:

public static extern void GetResult(IntPtr handle, [MarshalAs(UnmanagedType.SafeArray, SafeArraySubType = VarEnum.VT_USERDEFINED)] TestStruct[] data, ref int size);

Answer 1

Full example of use of a SAFEARRAY(int) C#->C++->C# (so the array is initialized with some data in C#, passed to C++, modified there and returned to C#).

C++:

// For the various _t classes for handling BSTR and IUnknown
#include <comdef.h>

struct ManagedUDT
{
    BSTR m_str01;
    int m_int01;

    ~ManagedUDT()
    {
        ::SysFreeString(m_str01);
        m_str01 = NULL;
    }
};

extern "C" __declspec(dllexport) void GetResult(SAFEARRAY*& data)
{
    if (data != NULL)
    {
        // Begin print content of SAFEARRAY
        VARTYPE vt;
        HRESULT hr = SafeArrayGetVartype(data, &vt);

        if (SUCCEEDED(hr))
        {
            // To make this code simple, we print only
            // SAFEARRAY(VT_I4)
            if (vt == VT_I4)
            {
                int *pVals;
                hr = SafeArrayAccessData(data, (void**)&pVals); // direct access to SA memory

                if (SUCCEEDED(hr))
                {
                    long lowerBound, upperBound;  // get array bounds
                    SafeArrayGetLBound(data, 1, &lowerBound);
                    SafeArrayGetUBound(data, 1, &upperBound);

                    long cnt_elements = upperBound - lowerBound + 1;

                    for (int i = 0; i < cnt_elements; i++)  // iterate through returned values
                    {
                        int val = pVals[i];
                        printf("C++: %d\n", val);
                    }

                    SafeArrayUnaccessData(data);
                }
                else
                {
                    // Error
                }
            }
        }
        else
        {
            // Error
        }
        // End print content of SAFEARRAY

        // Delete the SAFEARRAY if already present
        SafeArrayDestroy(data);
        data = NULL;
    }

    {
        // Creation of a new SAFEARRAY
        SAFEARRAYBOUND bounds;
        bounds.lLbound = 0;
        bounds.cElements = 10;

        data = SafeArrayCreate(VT_I4, 1, &bounds);
        int *pVals;

        HRESULT hr = SafeArrayAccessData(data, (void**)&pVals); // direct access to SA memory

        if (SUCCEEDED(hr))
        {
            for (ULONG i = 0; i < bounds.cElements; i++)
            {
                pVals[i] = i + 100;
            }
        }
        else
        {
            // Error
        }
    }
}

C#

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
private static extern void GetResult([MarshalAs(UnmanagedType.SafeArray, SafeArraySubType = VarEnum.VT_I4)] ref int[] ar);

and

var data = new int[] { 1, 2, 3, 4, 5 };
GetResult(ref data);

if (data != null)
{
    for (int i = 0; i < data.Length; i++)
    {
        Console.WriteLine("C#: {0}", data[i]);
    }
}
else
{
    Console.WriteLine("C#: data is null");
}

Code partially taken from https://stackoverflow.com/a/12484259/613130 and https://stackoverflow.com/a/3735438/613130

---

SAFEARRAY(VT_RECORD)

It is doable... Very hard... but doable. Please don't do it. You can't hate enough the world to do it. I do hope you don't!

C++:

// For the _com_util
#include <comdef.h>

extern "C"
{
    __declspec(dllexport) void GetResultSafeArray(SAFEARRAY *&psa)
    {
        // All the various hr results should be checked!
        HRESULT hr;

        // Begin sanity checks
        if (psa == NULL)
        {
            // Error
        }

        VARTYPE pvt;
        hr = ::SafeArrayGetVartype(psa, &pvt);

        if (pvt != VT_RECORD)
        {
            // Error
        }

        UINT size;
        size = ::SafeArrayGetElemsize(psa);

        if (size != sizeof(ManagedUDT))
        {
            // Error
        }

        // From tests done, it seems SafeArrayGetRecordInfo does a AddRef
        _com_ptr_t<_com_IIID<IRecordInfo, NULL> > prinfo;
        // The_com_ptr_t<>::operator& is overloaded
        hr = ::SafeArrayGetRecordInfo(psa, &prinfo);

        // From tests done, it seems GetName returns a new instance of the
        // BSTR
        // It is ok to use _bstr_t.GetAddress() here, see its description
        _bstr_t name1;
        hr = prinfo->GetName(name1.GetAddress());

        const _bstr_t name2 = _bstr_t(L"ManagedUDT");

        if (name1 != name2)
        {
            // Error
        }

        // End sanity checks

        long lowerBound, upperBound;  // get array bounds
        hr = ::SafeArrayGetLBound(psa, 1, &lowerBound);
        hr = ::SafeArrayGetUBound(psa, 1, &upperBound);
        long cnt_elements = upperBound - lowerBound + 1;

        // Begin print
        ManagedUDT *pVals;
        hr = ::SafeArrayAccessData(psa, (void**)&pVals);

        printf("C++:\n");

        for (int i = 0; i < cnt_elements; ++i)
        {
            ManagedUDT *pVal = pVals + i;

            // If you are using a recent VisualC++, you can
            // #include <memory>, and then
            //std::unique_ptr<char[]> pstr(_com_util::ConvertBSTRToString(pVal->m_str01));
            // and you don't need the char *pstr line and the delete[]
            // line
            char *pstr = _com_util::ConvertBSTRToString(pVal->m_str01);
            printf("%s, %d\n", pstr, pVal->m_int01);
            delete[] pstr;
        }

        hr = ::SafeArrayUnaccessData(psa);
        // End print

        // Begin free
        SAFEARRAYBOUND sab;
        sab.lLbound = 0;
        sab.cElements = 0;

        // SafeArrayRedim will call IRecordInfo::RecordClear
        hr = ::SafeArrayRedim(psa, &sab);
        // End Free

        // Begin create
        int numElements = 10;
        sab.cElements = numElements;
        hr = ::SafeArrayRedim(psa, &sab);

        hr = ::SafeArrayAccessData(psa, (void**)&pVals);

        for (int i = 0; i < numElements; i++)
        {
            ManagedUDT *pVal = pVals + i;

            char pstr[100];
            sprintf(pstr, "Element #%d", i);
            pVal->m_str01 = _com_util::ConvertStringToBSTR(pstr);

            pVal->m_int01 = 100 + i;
        }

        hr = ::SafeArrayUnaccessData(psa);
        // End create
    }

    __declspec(dllexport) void GetResultSafeArrayOut(SAFEARRAY *&psa, ITypeInfo *itypeinfo)
    {
        // All the various hr results should be checked!
        HRESULT hr;

        // Begin sanity checks
        if (psa != NULL)
        {
            // Begin free
            // SafeArrayDestroy will call IRecordInfo::RecordClear
            // if necessary
            hr = ::SafeArrayDestroy(psa);
            // End Free
        }

        // Begin create
        int numElements = 10;

        SAFEARRAYBOUND sab;
        sab.lLbound = 0;
        sab.cElements = numElements;

        // The_com_ptr_t<>::operator& is overloaded
        _com_ptr_t<_com_IIID<IRecordInfo, NULL> > prinfo;
        hr = ::GetRecordInfoFromTypeInfo(itypeinfo, &prinfo);

        psa = ::SafeArrayCreateVectorEx(VT_RECORD, 0, numElements, prinfo);

        ManagedUDT *pVals;
        hr = ::SafeArrayAccessData(psa, (void**)&pVals);

        for (int i = 0; i < numElements; i++)
        {
            ManagedUDT *pVal = pVals + i;

            char pstr[100];
            sprintf(pstr, "Element #%d", i);
            pVal->m_str01 = _com_util::ConvertStringToBSTR(pstr);

            pVal->m_int01 = 100 + i;
        }

        hr = ::SafeArrayUnaccessData(psa);
        // End create
    }
}

C#:

[ComVisible(true)]
[Guid("BBFE1092-A90C-4b6d-B279-CBA28B9EDDFA")]
[StructLayout(LayoutKind.Sequential)]
public struct ManagedUDT
{
    [MarshalAs(UnmanagedType.BStr)]
    public string m_str01;
    public Int32 m_int01;
}

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
static extern void GetResultSafeArray([MarshalAs(UnmanagedType.SafeArray)] ref ManagedUDT[] array);

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
static extern void GetResultSafeArrayOut([MarshalAs(UnmanagedType.SafeArray)] out ManagedUDT[] array, IntPtr itypeinfo);

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi, EntryPoint = "GetResultSafeArrayOut")]
static extern void GetResultSafeArrayRef([MarshalAs(UnmanagedType.SafeArray)] ref ManagedUDT[] array, IntPtr itypeinfo);

and

var arr = new[]
{
    new ManagedUDT { m_str01 = "Foo", m_int01 = 1},
    new ManagedUDT { m_str01 = "Bar", m_int01 = 2},
};

{
    Console.WriteLine("C#:");

    for (int i = 0; i < arr.Length; i++)
    {
        Console.WriteLine("{0}, {1}", arr[i].m_str01, arr[i].m_int01);
    }
}

{
    Console.WriteLine();

    var arr2 = (ManagedUDT[])arr.Clone();

    GetResultSafeArray(ref arr2);

    Console.WriteLine();

    Console.WriteLine("C#:");

    for (int i = 0; i < arr2.Length; i++)
    {
        Console.WriteLine("{0}, {1}", arr2[i].m_str01, arr2[i].m_int01);
    }
}

{
    Console.WriteLine();

    ManagedUDT[] arr2;

    IntPtr itypeinfo = Marshal.GetITypeInfoForType(typeof(ManagedUDT));
    GetResultSafeArrayOut(out arr2, itypeinfo);

    Console.WriteLine();

    Console.WriteLine("C#:");

    for (int i = 0; i < arr2.Length; i++)
    {
        Console.WriteLine("{0}, {1}", arr2[i].m_str01, arr2[i].m_int01);
    }
}

{
    Console.WriteLine();

    var arr2 = (ManagedUDT[])arr.Clone();

    IntPtr itypeinfo = Marshal.GetITypeInfoForType(typeof(ManagedUDT));
    GetResultSafeArrayRef(ref arr2, itypeinfo);

    Console.WriteLine();

    Console.WriteLine("C#:");

    for (int i = 0; i < arr2.Length; i++)
    {
        Console.WriteLine("{0}, {1}", arr2[i].m_str01, arr2[i].m_int01);
    }
}

There is a single big caveat for GetResultSafeArray: you must pass from C# at least an empty array (like a new ManagedUDT[0]). This because to create a SAFEARRAY(ManagedUDT) from nothing in C++ you would need a IRecordInfo object. I don't know how to retrieve it from C++. If you already have a SAFEARRAY(ManagedUDT) then clearly it has the IRecordInfo already set, so there is no problem. In the example given, in C++ there are first some sanity checks, then the passed array is printed, then it is emptied, then it is re-filled. The GetResultSafeArrayOut/GetResultSafeArrayRef "cheat": they receive from C# a ITypeInfo pointer (that is easy to retrieve in C#, with Marshal.GetITypeInfoForType()), and from taht the C++ can retrieve the IRecordInfo interface.

Some notes:

• I wrote Ansi-charset-C++. Normally for myself I always write Unicode-ready C++ (or directy Unicode-C++, because all the Windows NT support Unicode), but I've noticed that I'm an exception... So in various parts of the code there are conversions BSTR->Ansi->BSTR.

• I'm retrieving the HRESULT of all the function calls. They should be checked, and the failure handled.

• The most complex thing in C++/COM is knowing when to free something... In general always free/Release() everything! (be it BSTR/IUnknown derived interfaces, ...)

• Unless there is a bug, there is no support for this code. Consider it to be a proof of concept. I already lost various hours on it out of curiosity. You break it, you repair it.