COM interop - pass an [in, out] array

Question

Hello, I want to call FileStream.Read from COM Interop. The .Read method wants an [in, out] byte[] buffer to write to. What parameter and variant type should I pass to .invoke?
I tried passing a safearray of bytes with VT_ARRAY+VT_UI1. The .Read call succeeds and returns the number of bytes requested, but the buffer is not written to. Thanks

Accepted Answer

If you want to use IDispatch, try the following example ( use the debugger to examine the LPBYTE pointer after Invoke returns) -

SAFEARRAY *pByteArray = CreateEmptyByteArray(16);

DISPID did{ 0 };
DISPPARAMS dispparams = { nullptr, nullptr, 0, 0 };
PWSTR pszMethod = L"PassByteArray";
HRESULT hr = ptrTest->GetIDsOfNames(IID_NULL, &pszMethod, 1, LOCALE_USER_DEFAULT, &did);
if (SUCCEEDED(hr))
{
    VARIANT vByteArray, vResult;

    VariantInit(&vByteArray);
    VariantInit(&vResult);

    V_VT(&vByteArray) = VT_ARRAY | VT_BYREF | VT_UI1;
    V_ARRAYREF(&vByteArray) = &pByteArray;

    dispparams.rgvarg = &vByteArray;
    dispparams.cArgs = 1;

    hr = ptrTest->Invoke(did, IID_NULL, LOCALE_USER_DEFAULT, DISPATCH_METHOD, &dispparams, &vResult, nullptr, nullptr);
    if (SUCCEEDED(hr))
    {
        LPBYTE pBytes = nullptr;
        hr = SafeArrayAccessData(pByteArray, (LPVOID*) &pBytes);
        hr = SafeArrayUnaccessData(pByteArray);
    }

}

SafeArrayDestroy(pByteArray);

Answer

To pass a buffer from unmanaged code to a managed COM Server that wants a read/write byte[] array in the managed code you can refer to the following example.

C# COM Server method -

public void PassByteArray(ref byte[] bArr)
{
    int len = bArr.Length;
    bool ro = bArr.IsReadOnly;
    for (int x = 0; x < len; x++)
        bArr[x] = 0xFF;
}

The above example just writes some data to be buffer it receives from the unmanaged code.

Example C++ code (no error checking) to create SAFEARRAY for buffer and pass it to managed COM Server -

SAFEARRAY * CreateEmptyByteArray(int nBytes)
{
    ULONG nElements = nBytes;
    SAFEARRAYBOUND bound = { nElements, 0 };

    return SafeArrayCreate(VT_UI1, 1, &bound);
}

-----------------------------------------

SAFEARRAY *pByteArray = CreateEmptyByteArray(16);

hr = ptrTest->PassByteArray(&pByteArray);

After the call to the COM method returns you can see that the SAFEARRAY reflects the changes made by the COM Server.

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COM interop - pass an [in, out] array

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