Passing string arrays from COM to C#

Question

I need to access C# methods in a COM dll via COM-like interface. One of the method requires an array of strings to be passed as input.

Am creating a SAFEARRAY and passing it to the COM Interop. However, this does not seem to be working, as I see an exception in the interop layer. (System.Runtime.InteropServices.SafeArrayTypeMismatchException).

Obviously, there seems to be a difference in type being expected.

Pasting the code here:

C# method to be invoked:

public long DoIt3(int nFiles, string[] fileNames);

C++ code invoking the same:

int _tmain()
{
TCHAR *fileNames[128] = { TEXT("C:\\Program Files\\IBM\\RTC.NET"),
                          TEXT("C:\\KIRAN\\Work\\RFT"), TEXT(".\\bin\\Debug") };

SAFEARRAY *pSA = CreateSafeStringArray(3, fileNames);

_tprintf(TEXT("%d"), pIManaged->DoIt3(3, pSA));

SafeArrayDestroy(pSA);
}

static SAFEARRAY *CreateSafeStringArray(long nElements, TCHAR *elements[])
{
SAFEARRAYBOUND saBound[1];

saBound[0].cElements = nElements;
saBound[0].lLbound = 0;

SAFEARRAY *pSA = SafeArrayCreate(VT_VARIANT, 1, saBound);

if (pSA == NULL)
{
    return NULL;
}

for (int ix = 0; ix < nElements; ix++)
{
    VARIANT v;

    VariantInit(&v);

    v.vt = VT_BSTR;
    v.bstrVal = elements[ix];

    long rgIndicies[1];

    rgIndicies[0] = ix + saBound[0].lLbound;

    HRESULT hr = SafeArrayPutElement(pSA, rgIndicies, &v);

    _tprintf(TEXT("%d"), hr);

    VariantClear(&v);
}

return pSA;
}

Any ideas/suggestions are welcome.

Answer 1

I figured it out! The following code works:

static SAFEARRAY *CreateSafeStringArray(long nElements, TCHAR *elements[])
{
SAFEARRAYBOUND saBound[1];

saBound[0].cElements = nElements;
saBound[0].lLbound = 0;

SAFEARRAY *pSA = SafeArrayCreate(VT_BSTR, 1, saBound);

if (pSA == NULL)
{
    return NULL;
}

for (int ix = 0; ix < nElements; ix++)
{
    BSTR pData = SysAllocString(elements[ix]);

    long rgIndicies[1];

    rgIndicies[0] = saBound[0].lLbound + ix;

    HRESULT hr = SafeArrayPutElement(pSA, rgIndicies, pData);

    _tprintf(TEXT("%d"), hr);
}

return pSA;
}

Thanks for all your suggestions!

Answer 2

In the case of an array of BSTR strings, you can set the BSTR values directly on your array, and also you need to allocate memory for your BSTR elements, you could use ATL/MFC CString for that:

...
psa = SafeArrayCreate( VT_BSTR, 1, saBound);
HRESULT hr = SafeArrayLock( psa );
//TODO: test for hr success

if (pSA == NULL)
{
    return NULL;
}

for (int ix = 0; ix < nElements; ix++)
{
    long rgIndicies[1];
    rgIndicies[0] = ix + saBound[0].lLbound;
    CString tempstr(elements[ix]);

    ((BSTR*)psa->pvData)[ix] = tempstr.AllocSysString();
    _tprintf(TEXT("%d"), hr);
}

hr = SafeArrayUnlock( psa );
//TODO: test for hr success
...