COM返回值

interface IImageService : IDispatch{
    [id(1), helpstring("方法QueryImage")] HRESULT QueryImage([out] VARIANT* imagedata, [out,retval] VARIANT* imglength);
};

STDMETHODIMP CImageService::QueryImage(VARIANT* imagedata, VARIANT* imglength)
{
    // TODO: 在此添加实现代码
    FILE *file = fopen("d:\\test.png","rb");
    if (file == NULL) return S_FALSE;
    fseek(file, 0, SEEK_END);
    fpos_t pos;
    fgetpos(file, &pos);
    fseek(file, 0, SEEK_SET);
    int length = (int)pos;

    VariantInit(imglength);
    imglength->vt = VT_I4;
    imglength->lVal = length;
   
    VariantInit(imagedata);
    SAFEARRAY *psa = SafeArrayCreateVector(VT_UI1, 0, length); //unsigned char

    fread(psa->pvData, 1, length, file);
    fclose(file);

    imagedata->vt = VT_ARRAY | VT_UI1;
    imagedata->parray = psa;

    return S_OK;
}

C#调用：

            object imagedata;
            object imglen = imgService.QueryImage(out imagedata);

            byte[] buffer = (byte[])imagedata;
            int len = (int)imglen;