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 following:



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 textbox:

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

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

	return S_OK;
}

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

// 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 =
{0xC8F6E230,0x2672,0x11D3,{0xA8,0xA8,0x00,0x10,0x5A,0xA9,0x43,0xDF}};

const CLSID CLSID_First_ATL =
{0x970599E0,0x2673,0x11D3,{0xA8,0xA8,0x00,0x10,0x5A,0xA9,0x43,0xDF}};

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
	if(SUCCEEDED(hr))
	{
		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.
		if(SUCCEEDED(hr))
		{
			long ReturnValue;

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

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