Microsoft Office Excel 2002 and Microsoft Office 2007 support Automation Add-ins in addition to Component Object Model (COM) Add-ins. This article explains the differences between these two types of Add-ins.

COM Add-ins

COM Add-ins present the developer with a way to extend the functionality of Office 2000, Office XP, Office 2003 and Office 2007 applications for custom tasks. COM Add-ins are typically used to automate Excel in response to a click of a CommandBar button, a form or dialog box, or some other event specific to Excel such as opening or closing workbooks or entering data on worksheets. COM Add-in functions cannot be directly called from cell formulas in worksheets.

A COM Add-in is an in-process COM server (an ActiveX DLL) that must implement the IDTExensibility2 interface. All COM Add-ins must implement each of the five methods of this interface: OnConnection, OnStartupComplete, OnAddinsUpdate, OnBeginShutDown, and OnDisconnection.

When a COM Add-in is installed on a user's system, registry entries are created for the Add-in. In addition to normal COM registration, a COM Add-in is registered for each Office application in which it runs. COM Add-ins used by Excel are registered in the following registry key: This key contains a subkey for each installed COM Add-in. The name of the subkey is the ProgID for the COM Add-in. The subkey for a COM Add-in also contains values that describe the COM Add-in's friendly name, description, and load behavior. The load behavior describes how the Add-in is loaded in Excel: loaded at startup, loaded at next startup only, loaded on demand, or not loaded.

COM Add-ins can also be loaded and unloaded through the Excel 2002 user interface. To do this, follow these steps:

1.	On the View menu, point to Toolbars and then click Customize.
2.	In the Toolbars dialog box, click the Advanced tab. In the list of categories, select Tools. Locate COM Add-ins in the list of commands and drag the command to a menu or CommandBar of your choice. Close the Toolbars dialog box.
3.	Click the COM Add-ins command that you added to display the COM Add-ins dialog box. The dialog box lists all COM Add-ins that are installed on your system, and the COM Add-ins that are currently loaded are selected.

COM Add-ins can also be loaded and unloaded through the Excel 2007 user interface. To do this, follow these steps:

1.	Click the Microsoft Office Button, and then click Excel Options.
2.	Click Add-Ins.
3.	Under Manage, click COM Add ins, and then click Go . The COM Add-ins dialog box lists all COM add-ins that are installed on the computer. The COM add-ins that are currently loaded are selected.

For additional information about COM Add-ins, click the following article numbers to view the articles in the Microsoft Knowledge Base: For more information, see the following Microsoft Web site:

Automation Add-ins

In addition to COM Add-ins, Excel 2002, and Excel 2003 supports Automation Add-ins. Automation Add-ins build on COM Add-ins in that functions in Automation Add-ins can be called from formulas in Excel worksheets. COM Add-ins must be in-process COM servers that support the IDTExtensibility2 interface; however, Automation Add-ins can be in-process or out-of-process COM servers and implementation of IDTExtensibility2 is optional.

To use functions from an Automation Add-in in Excel, follow these steps:

1.	On the Tools menu, click Add-Ins.
2.	In the Add-Ins dialog box, click Automation. From the list of registered COM servers, select your Automation Add-in and click OK.
3.	The Automation Add-in appears in the Add-ins dialog box. Click OK to close the Add-Ins dialog box.

To use functions from an Automation Add-in in Excel 2007, follow these steps:

1.	Click the Microsoft Office Button, and then click Excel Options.
2.	Click Add-Ins.
3.	Under Manage, click Excel Add ins, and then click Go.
4.	In the Add-Ins dialog box, click Automation. In the list of registered COM servers, click your Automation Add-in, and then click OK. The Automation Add-in appears in the Add-ins dialog box. Click OK to close the Add-Ins dialog box.

When you make additions to the list in the Add-Ins dialog box or when you select and clear Add-ins in the list, Excel stores your changes in the registry. First, Excel uses the following registry setting to determine whether or not an Automation Add-in in the Add-in list is loaded: The /A switch that is used in the string value is new to Excel 2002 / Excel 2003 and is used specifically to load Automation Add-ins. All Automation Add-ins are loaded on demand; there is no setting that can change the load behavior for an Automation Add-in.

When an Automation Add-in that is listed in the Add-Ins dialog box is cleared, a subkey with a name equal to the Add-in's ProgID is created in the following registry key: This registry setting ensures that Automation Add-ins that you have added to the Add-ins list are retained in the list even when you have chosen not to load them.

For more information about Automation Add-Ins, see the following articles in the Microsoft Knowledge Base: For more information, see the following Microsoft Web site:

Automation Add-ins That Implement IDTExtensibility2

As previously mentioned, an Automation Add-in may implement IDTExtensibility2, but it is not required in order for Excel to call the functions in the Add-in from a worksheet. If you require that your Automation Add-in obtains a reference to the Excel instance, you can implement IDTExtensibility2 and use the Application parameter of OnConnection to automate Excel.

An Automation Add-in that implements IDTExtensibility2 can be loaded in the Excel user interface through both the COM Add-Ins dialog box and the Add-Ins dialog box. The following describes the behavior of an Automation Add-in based on whether it is loaded in one or both of these dialog boxes:

•	Loaded only in the Add-ins dialog box. The Add-in is loaded on demand. Functions in the Add-in may be called from formulas in a worksheet.
•	Loaded only in the COM Add-ins dialog box. The Add-in is loaded as a COM Add-in and its load behavior is determined from settings in the registry. Functions in the Add-in cannot be called from formulas in a worksheet.
•	Loaded in both the COM Add-ins dialog box and the Add-ins dialog box. Two separate instances of the Add-in are loaded. One instance is loaded as a COM Add-in and the other instance is loaded as an Automation Add-in. The COM Add-in instance uses the load behavior indicated in the registry; the Automation Add-in instance loads on demand. The two instances work independently of one another and do not share global variables.

Because Automation Add-ins are loaded on demand, Excel may attempt to load the Add-in while it is in cell edit mode. Therefore, when you develop an Automation Add-in that supports IDTExtensibility2, you should be careful not to do anything that attempts to change Excel's state while the Add-in loads. For more information, see the following article in the Microsoft Knowledge Base:
(c) Microsoft Corporation 2001, All Rights Reserved. Contributions by Lori B. Turner, Microsoft Corporation.