Getting the most out of IDispatch(XYDispDriver)

Introduction

Don't be fooled by the title. Even if you are not (will not be) dealing with the

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

Here are some of the public methods of

XYDispDriver

. Please note the changes from its previous version.


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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).

Sample code

The following code demonstrates how to use

XYDispDriver

to create a
com object and call its methods in a console application.


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#include "XYDispDriver.h"

void main()
{
// declare the dispatch driver

XYDispDriverdisp;
// 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.

Other Features

You can use

XYDispDriver

with COM+ to create
com object on a remote machine. Here is what you have to do.

Register the com dll or exe on the remote machine. Create a
COM+ "Server" application and add this
component.
Export the COM+ application from the remote machine (generate a .MSI file).

Import the COM+ application to your local machine (run the .MSI file).

Call the

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.

The name of the com method.
Invocation flag (

DISPATCH_METHOD

,

DISPATCH_PROPERTYGET

, or

DISPATCH_PROPERTYPUT

).
Number of parameters for the com method.

List of parameters (the parameter values have to be in

VARIANT

variables).

Thank you for reading this article, please refer to my
home page for other articles and programs.

Recent Updates

12/11/2003 - Modified article text and code to create
com object on a remote machine.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found
here

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