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Getting the most out of IDispatchBy Xiangyang Liu 刘向阳A C++ class that makes it extremely easy to use a COM object, even in console apps |
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IDispatch interface directly, this article and the included source code could be very useful to you. In my previous article "Buried treasure in MFC: COleDispatchDriver", I introduced XYDispDriver, a class derived from COleDispatchDriver, that can be used to dynamically create and use com objects. The advantage of XYDispDriver is its simple interface. You create and use all (well, almost all) com objects the same way. There is no need for wrapper classes. You don't even need type libraries at compile time.
Then I realized that not all people love MFC as I do. By diving into the source code of COleDispatchDriver, I found out that there is really no need to get MFC involved. I have re-coded the class XYDispDriver from scratch so that it does not depend on any library (sorry, ATL and STL fans :-).
Most of my code is borrowed from MFC, of course. Fortunately, XYDispDriver without MFC is still simple and easy to use.
XYDispDriver. Please note the changes from its previous version. class XYDispDriver { ... public: XYDispDriver(); ~XYDispDriver(); // create a com object with given prog id or class id bool CreateObject(LPCTSTR strProgID, DWORD dwClsContext = CLSCTX_ALL, LPCTSTR strServerName = NULL); bool CreateObject(CLSID clsid, DWORD dwClsContext = CLSCTX_ALL, LPCTSTR strServerName = NULL); // get the type of a property VARTYPE GetPropertyType(LPCTSTR strPropertyName); // get the property value VARIANT* GetProperty(LPCTSTR strPropertyName); // set the property value bool SetProperty(LPCTSTR strPropertyName, ...); // get return type of a method VARTYPE GetReturnType(LPCTSTR strMethodName); // get number of parameters in a method int GetParamCount(LPCTSTR strMethodName); // get the type of a parameter in a method VARTYPE GetParamType(LPCTSTR strMethodName, const int nParamIndex); // invoke a method VARIANT* InvokeMethod(LPCTSTR strMethodName, ...); // get the last error code as HRESULT HRESULT GetLastError() { return m_hRet; } // get exception info EXCEPINFO* GetExceptionInfo() { return m_pExceptInfo; } };
The CreateObject functions create a com object: getting a pointer to its IDispatch interface and also querying its ITypeInfo interface to load all the necessary type information. This makes it possible to use the InvokeMethod function to call various com methods in an incredibly flexible way.
The output values of GetProperty and InvokeMethod are stored in internal variables of type VARIANT. These methods will return a pointer to VARIANT or NULL if unsuccessful (for example, the method or property with given name does not exist, or there is a com error). If you don't store the return value, it will be overridden when the same method (or property) is invoked the next time. There is no need to clean up memory for the internal variables. To use XYDispDriver in a multi-threaded process, you can either use a separate object in each different thread or serialize the access to shared objects using critical sections. Please note that the InvokeMethod function can take a variable number of arguments.
For power users, the GetLastError returns the last SCODE and the GetExceptionInfo returns a pointer to an EXCEPINFO structure (maybe NULL if there is no exception).
XYDispDriver to create a com object and call its methods in a console application. #include "XYDispDriver.h" void main() { // declare the dispatch driver XYDispDriver disp; // create the com object from its prog id if(disp.CreateObject("XYDBREADER.XYDBReaderCtrl.1")) { // call the OpenDB method which takes 3 string arguments and returns // a bool VARIANT* pOutput = disp.InvokeMethod("OpenDB","XY_TAG","",""); if(pOutput&&pOutput->boolVal) { printf("Database opened\n"); // call the OpenQuery method which takes a long and a string // arguments and returns a long pOutput = disp.InvokeMethod("OpenQuery",0, "select * from XYTagName"); if(pOutput&&pOutput->lVal) { printf("Query opened: %d\n",pOutput->lVal); long nQueryID = pOutput->lVal; // call the GetAllRec method which takes a long argument // and returns a string (BSTR) pOutput = disp.InvokeMethod("GetAllRec",nQueryID); if(pOutput) { printf("Got data"); // print the output data wprintf(L"Returned data:\n%s\n",pOutput->bstrVal); // call the CloseQuery method which takes a long argument // and returns nothing disp.InvokeMethod("CloseQuery",nQueryID); } } // call the CloseDB method which takes no argument and returns // nothing disp.InvokeMethod("CloseDB"); } } printf("done\n"); }
It is also possible to use XYDispDriver with existing com objects. All you need to do is declare an instance of XYDispDriver and call the Attach method (not listed above) passing the IDispatch pointer of the existing com object.
You can use XYDispDriver with COM+ to create com object on a remote machine. Here is what you have to do.
CreateObject method of XYDispDriver passing the name of the remote machine as the third parameter.The XYDispDriver retrieves type info of a com object it created and uses the type info to call com methods dynamically. Sometimes it is not possible to retrieve type info or not efficient to do so. There is an InvokeVariantMethod method (not listed above) in the XYDispClass class which can be used to invoke almost any com method as long as we have the IDispatch pointer of the com object. To use this method, you need to provide the following.
IDispatch pointer of a com object.
DISPATCH_METHOD, DISPATCH_PROPERTYGET, or DISPATCH_PROPERTYPUT).
VARIANT variables).Thank you for reading this article, please refer to my home page for other articles and programs.
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Last Updated: 11 Dec 2003 Editor: Nishant Sivakumar |
Copyright 2001 by Xiangyang Liu 刘向阳 Everything else Copyright © CodeProject, 1999-2009 Web18 | Advertise on the Code Project |
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What I'm confused is that the VT_USERDEFINED doesn't seems to be present in