A Simple COM tutorial using ATL

Environment: VC6 SP3, VB6 SP3, NT4 SP4, Windows 2000 Beta 3

The purpose of this tutorial is to give you an idea on how to
create a COM Server using ATL, and then being able to call the
server from both a Visual C++ program, and a Visual Basic program.
I am not gonna go into the depths of COM details or burden you down
with IDL, this tuorial is designed to show a new VC++ programmer,
how easy “Simple” COM objects are to create using ATL, and to whet
their appetite for wanting to learn more.

Step 1: Running the ATL COM Wizard

The first thing you need to do is to fire up Visual C++ and create a new
project. Choose the “ATL COM AppWizard”. In the project name call
it “Simple_ATL”. Set the location where you want this project to
be saved in, then hit the Ok button. You will see a screen that
gives you several choices. The first choice is “Server Type”.
We are going to build a Server DLL, so make sure that the Server
Type is set to “Dynamic Link Library”. The other three checkboxes
below do not concern us for this particular project, so we can ignore
them. Press the finish button to have the Wizard generate to
appropriate files for you. A “New Project Information” window will
appear to tell you what files are going to be created. Press the Ok
button to accept this.

Step 2: Creating a new ATL object

Make sure you can see the “Workspace View” inside the VC++ IDE.
You can do this by clicking the “View” menu, then choosing “Workspace”.
There will be three tabs, click on the “ClassView” tab. You should see
“Simple_ATL Classes”. Right click on this and choose “New ATL
Object” from the popup menu. You will see a window like the

The default choice (Simple Object) is what we want. Click the next
button and you will be in the “ATL Object Wizard Properties”
window. In the “Short Name” textbox, enter “First_ATL”. Notice
how the Wizard automatically fills in the rest of the textboxes for
you. Click on the “Attributes” tab at the top. Here you have
several choices to make. For the first choice, Threading Model, we
will stick with the default Apartment Model. For the “Interface”,
click on “Custom”, instead of “Dual”. Finally, as we are not going
to be concerned with “Aggregation”, click on the “No” radio button.
We don’t need to worry about any of the three checkboxes at the
bottom. Click on the Ok button and let the Wizard create our new
ATL Simple Object.

Step 3: Adding a method

If you click on the “ClassView” tab now in your workspace, you will
notice that the Wizard added a bunch of things. The first thing we
want to do is add a method. We can do this easily by right
clicking on “IFirst_ATL” and choosing “Add Method”.

Once you have clicked on “Add Method” you will see the “Add Method
to Interface” window. Under the Return Type you can see that by
default the method will return “HRESULT”. In most cases you should
leave this as is. The next textbox allows us to type in the Method
Name. Lets type in “AddNumbers”. The last textbox asks us the
Parameters we wish to use. As we want to add two numbers together,
and get a result back, we will use three parameters. The last
parameter will be a pointer. Now without going into a 300 page
tutorial on IDL, we need to type in the following in the parameter

[in] long Num1, [in] long Num2, [out] long *ReturnVal

In a nutshell, we are declaring two parameters as long, the values
are going in [in], and a final value to return [out] the answer.
(It might looking kinda weird the first time you see it, but once
you read a book or two on COM, this will make more sense) Click on
the Ok button. Click on the “ClassView” tab and expand all the “+”
symbols so the tree is fully open to view. Under the top
interfaace (“IFirst_ATL”) you will see our “AddNumbers” method, and
the parameters we gave it. Double click on this, and it will place
you into the code. Add the following code:

STDMETHODIMP CFirst_ATL::AddNumbers(long Num1, long Num2, long *ReturnVal)
	// TODO: Add your implementation code here
	*ReturnVal = Num1 + Num2;

	return S_OK;

Step 4: Compiling the DLL

Believe it or not, but you have a working COM Server built with
ATL! Of course we need to compile it. To do this, press the “F7”
button and let VC++ do its work. The compiler will grind away for
a few seconds. The compiler will register your new DLL in the
registry so that other programs can make use of it. Lets try it

Step 5: Testing the COM Server with Visual Basic

To start with, we will use Visual Basic to test out the COM Server.
(If you do not have a copy of VB, you can skip ahead to the
section on testing the COM Server in VC++) Fire up VB and choose
“Standard EXE” as your project. Place a Command button on the
dialog. Now we need to add a reference to the COM Server. Click
on the “Project” menu and choose “References”. Scroll down until
you see “Simple ATL 1.0 Type Library” and click on it.

Click the Ok button. Now, double click on the command button you
placed earlier and VB will drop you into the code window for the
command button. Add the following code:

Private Sub Command1_Click()
    Dim objTestATL As SIMPLE_ATLLib.First_ATL
    Set objTestATL = New First_ATL

    Dim lngReturnValue As Long

    objTestATL.AddNumbers 5, 7, lngReturnValue

    MsgBox "The value of 5 + 7 is: " & lngReturnValue
End Sub

If your a VB programmer this should be pretty straight forward. We
declare and object, call the “AddNumbers” from the COM Server, then
display the results. Press the “F5” key to run to the VB project,
click on the command button, and you should see the expected

Not too hard. Lets try this again, except with VC++.

Step 6: Testing the COM Server with Visual C++

Save and close the Simple_ATL project if it is still open and
create a new project. Choose a “Win32 Console Application” and
name it “Test_ATL”. Click the Ok button and accept the default
(An empty project) for the next window. Click on the finish
button, then hit the Ok button again. You should now have an empty
project. Press the “Control” and “N” keys to add a file to this
project. From the window, choose “C++ Source File” and name it
“Test_ATL.cpp”. Press the Ok button to accept. You should have a
blank file open. We need to add some code now to test out the COM
Server. Start adding the following code to the new cpp file:

// You need to point this header file to the directory
// you placed the Simple_ATL project

#include "..\Simple_ATL\Simple_ATL.h"
#include <iostream.h>

// Copy the following from the Simple_ATL_i.c file
// from the Simple_ATL project directory

const IID IID_IFirst_ATL =

const CLSID CLSID_First_ATL =

void main(void)
	// Declare and HRESULT and a pointer to the Simple_ATL interface
	HRESULT			hr;
	IFirst_ATL		*IFirstATL;

	// Now we will intilize COM
	hr = CoInitialize(0);

	// Use the SUCCEEDED macro and see if we can get a pointer
	// to the interface
		hr = CoCreateInstance( CLSID_First_ATL, NULL, CLSCTX_INPROC_SERVER,
						IID_IFirst_ATL, (void**) &IFirstATL);

		// If we succeeded then call the AddNumbers method, if it failed
		// then display an appropriate message to the user.
			long ReturnValue;

			hr = IFirstATL->AddNumbers(5, 7, &ReturnValue);
			cout << "The answer for 5 + 7 is: " << ReturnValue << endl;
			hr = IFirstATL->Release();
			cout << "CoCreateInstance Failed." << endl;
	// Uninitialize COM

Step 7: Compile and run the program

Compile the program by pressing the "F5" key, then run it by
pressing the "Control" and "F5" keys. You should see a DOS window open
up and giving you the expected results.


Source for the server - 14.3 Kb

Source for the VB test project- 1.19 Kb

Source for the VC test project - 2.83 Kb


More by Author

Must Read