Cast from .NET to COM using Managed C++?

By Augusto Goncalves

After select an object using .NET (Managed C++), how cast to a COM object? First you need to get the IUnknow pointer from AcadObject property, then cast it to a COM object. This procedure will ensure you maintain COM reference count.

The following code show how do it, also show how get its type name. Keep in mind that this project should be created in Mixed mode.

//get the collection of documents (MDI)    Autodesk::AutoCAD::ApplicationServices::DocumentCollection ^docMgr =       Autodesk::AutoCAD::ApplicationServices::      Application::DocumentManager;    //get the active document    Autodesk::AutoCAD::ApplicationServices::Document ^acadDoc =      docMgr->MdiActiveDocument;    //get the editor    Autodesk::AutoCAD::EditorInput::Editor ^ed = acadDoc->Editor;         Autodesk::AutoCAD::DatabaseServices::Transaction ^trans;    try    {      //start a transaction      trans = acadDoc->Database->TransactionManager->        StartTransaction();           //select an entity using .NET      Autodesk::AutoCAD::EditorInput::PromptEntityResult        ^promptEntRes = ed->GetEntity(_T("Select an entity: "));      if (promptEntRes->Status != Autodesk::AutoCAD::        EditorInput::PromptStatus::OK)         return;           //open the entity for read      Autodesk::AutoCAD::DatabaseServices::Entity ^mgdEntity =         (Autodesk::AutoCAD::DatabaseServices::Entity^)        trans->GetObject(promptEntRes->ObjectId,         Autodesk::AutoCAD::DatabaseServices::OpenMode::ForRead);           //get the COM object from the .NET object      System::Object ^acadObj = mgdEntity->AcadObject;      System::IntPtr ptr = System::Runtime::        InteropServices::Marshal::GetIUnknownForObject(acadObj);      IUnknown* iunk = static_cast(ptr.ToPointer());      CComQIPtr acadEntity = iunk;      iunk->Release();           //get the entity name using the COM object      //it will return something line AcDbLine, AcDbCircle, etc      CComBSTR entityName;      acadEntity->get_EntityName(&entityName);      //if you made the proper cast, such as for IAcadLine,      //then you can call any other method of this COM object           //now get the COM interface name of the object      //it will return something line IAcadLine, IAcadCircle, etc      CComPtr typeInfo;      acadEntity->GetTypeInfo(0,LOCALE_SYSTEM_DEFAULT,&typeInfo);      CComBSTR interfaceName;      typeInfo->GetDocumentation(-1,&interfaceName, 0,0,0);           //and print using .NET      ed->WriteMessage(_T("nEntity selected: {0} ({1})"),         gcnew System::String(entityName),         gcnew System::String(interfaceName));    }    catch(System::Exception^ e)    {      //something went wrong      ed->WriteMessage(e->Message);    }    __finally    {      //dispose the transaction      trans->~Transaction();    }