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POProblems accessing a COM interface in C++
 

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  1. POProblems accessing a COM interface in C++
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    <p>What I want to do is access a COM interface and then call the "<strong>Open</strong>" method of that interface. I have a sample code in Visual Basic which works fine, but I need to write it in C++ and I can't seem to get it to work.</p> <p>First of all, this is the working VB code:</p> <pre><code>Dim CANapeApplication As CANAPELib.Application CANapeApplication = CreateObject("CANape.Application") Call CANapeApplication.Open("C:\Users\Public\Documents\Vector\CANape\12\Project", 0) </code></pre> <p><strong>CANape.Application</strong> is the ProgID which selects the interface I need.</p> <p>After reading some docs at msdn.microsoft.com and <a href="https://stackoverflow.com/questions/2313432/how-to-retrieve-the-interface-id-of-a-com-class-so-that-it-can-be-passed-to-cocr">this question</a>, I wrote this code: </p> <pre><code>void ErrorDescription(HRESULT hr); //Function to output a readable hr error int InitCOM(); int OpenCANape(); // Declarations of variables used. HRESULT hresult; void **canApeAppPtr; IDispatch *pdisp; CLSID ClassID; DISPID FAR dispid; UINT nArgErr; OLECHAR FAR* canApeWorkingDirectory = L"C:\\Users\\Public\\Documents\\Vector\\CANape\\12\\Project"; int main(){ // Instantiate CANape COM interface if (InitCOM() != 0) { std::cout &lt;&lt; "init error"; return 1; } // Open CANape if (OpenCANape() != 0) { std::cout &lt;&lt; "Failed to open CANape Project" &lt;&lt; std::endl; return 1; } CoUninitialize(); return 0; } void ErrorDescription(HRESULT hr) { if(FACILITY_WINDOWS == HRESULT_FACILITY(hr)) hr = HRESULT_CODE(hr); TCHAR* szErrMsg; if(FormatMessage( FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM, NULL, hr, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPTSTR)&amp;szErrMsg, 0, NULL) != 0) { _tprintf(TEXT("%s"), szErrMsg); LocalFree(szErrMsg); } else _tprintf( TEXT("[Could not find a description for error # %#x.]\n"), hr); } int InitCOM() { // Initialize OLE DLLs. hresult = OleInitialize(NULL); if (!SUCCEEDED(hresult)) { ErrorDescription(hresult); return 1; } // Get CLSID from ProgID //hresult = CLSIDFromProgID(OLESTR("CANape.Application"), &amp;ClassID); hresult = CLSIDFromProgID(OLESTR("CanapeCom.CanapeCom"), &amp;ClassID); if (!SUCCEEDED(hresult)) { ErrorDescription(hresult); return 1; } // OLE function CoCreateInstance starts application using GUID/CLSID hresult = CoCreateInstance(ClassID, NULL, CLSCTX_LOCAL_SERVER, IID_IDispatch, (void **)&amp;pdisp); if (!SUCCEEDED(hresult)) { ErrorDescription(hresult); return 1; } // Call QueryInterface to see if object supports IDispatch hresult = pdisp-&gt;QueryInterface(IID_IDispatch, (void **)&amp;pdisp); if (!SUCCEEDED(hresult)) { ErrorDescription(hresult); return 1; } std::cout &lt;&lt; "success" &lt;&lt; std::endl; return 0; } int OpenCANape() { //Method name OLECHAR *szMember = L"Open"; // Retrieve the dispatch identifier for the Open method // Use defaults where possible DISPID idFileExists; hresult = pdisp-&gt;GetIDsOfNames( IID_NULL, &amp;szMember, 1, LOCALE_SYSTEM_DEFAULT, &amp;idFileExists); if (!SUCCEEDED(hresult)) { std::cout &lt;&lt; "GetIDsOfNames: "; ErrorDescription(hresult); return 1; } unsigned int puArgErr = 0; VARIANT VarResult; VariantInit(&amp;VarResult); DISPPARAMS pParams; memset(&amp;pParams, 0, sizeof(DISPPARAMS)); pParams.cArgs = 2; VARIANT Arguments[2]; VariantInit(&amp;Arguments[0]); pParams.rgvarg = Arguments; pParams.cNamedArgs = 0; pParams.rgvarg[0].vt = VT_BSTR; pParams.rgvarg[0].bstrVal = SysAllocString(canApeWorkingDirectory); pParams.rgvarg[1].vt = VT_INT; pParams.rgvarg[1].intVal = 0; // debug mode // Invoke the method. Use defaults where possible. hresult = pdisp-&gt;Invoke( dispid, IID_NULL, LOCALE_SYSTEM_DEFAULT, DISPATCH_METHOD, &amp;pParams, &amp;VarResult, NULL, &amp;puArgErr ); SysFreeString(pParams.rgvarg[0].bstrVal); if (!SUCCEEDED(hresult)) { ErrorDescription(hresult); return 1; } return 0; } </code></pre> <p>There are several problems with this. </p> <ul> <li>Using the ClassID received from <strong>CLSIDFromProgID</strong> as the first parameter of <strong>CoCreateInstance</strong> does not work, it returns the error: <strong>class not registered</strong></li> <li>If i use the ProgID <strong>CanapeCom.CanapeCom</strong> (I found it by looking in the Registry), <strong>CoCreateInstance</strong> works. However, when I use <strong>pdisp->GetIDsOfNames</strong> I get the error message: <strong>Unkown name</strong>. Which I think means that the method was not found. That seems logical because I've used a different ProgID, but I just can't figure out how to get to the interface I'm looking for.</li> <li>I have also tried to use the resulting CLSID from <code>CLSIDFromProgID(OLESTR("CANape.Application"), &amp;ClassID);</code> as the 4th argument of <strong>CoCreateInstance</strong> but that resulted in a <strong>"No such interface supported"</strong> error.</li> </ul> <p>Do I need the dll file of the software? In the VB example the dll file is used to get the interface and then create a new object using the ProgID. I'm not sure if I need to do the same in C++ or how this should work.</p> <p>I'm really stuck here and hope that someone can help me.</p>
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    1. COUsually you just let some tool generate a wrapper around the COM library in C++ and use that. Try `#import "CANape.tlb"` if you're using Visual Studio.
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      1. POProblems accessing a COM interface in C++
      1. USAlex
    2. COCheck you're not making a 64 bit build as presumably CanApe is only 32 bit.
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      1. POProblems accessing a COM interface in C++
      1. USJonathan Potter
    3. COFollowing on Jonathan's comment, if that is the case, you *can* make it work using COM surrogates, but don't expect it to be trivial (and honestly, I don't recc it).
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      1. USWhozCraig
 

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