Understanding the Forms Cache

The forms cache is a folder located on a computer's hard disk that stores a local copy of a standard Outlook form. The forms cache does not cache form regions because these form components are already stored on the computer's hard disk.

The forms cache improves the load time of a form because commonly used forms are loaded from the hard disk rather than downloaded from the server. When a form is activated for the first time, the form definition file is copied from its forms library to the Forms folder. The forms cache keeps a temporary copy of the form definition in a subfolder. This subfolder's name roughly matches the name of the form.

The form table, Frmcache.dat, also located in the Forms folder, is used to locate a form and to prevent multiple instances of the same form from being loaded in the cache. When a form is activated, Microsoft Outlook checks to see if a form with the same message class is already in the cache. If not, it copies the form definition to the cache. In addition, if a change has been made to a form, Outlook copies the new form definition to the cache.

Microsoft Office Outlook 2007 looks for forms in the following order. When a match is found, Outlook opens the form and does not search further.

1. Forms cached in memory. If you have another item open that uses the same form, Outlook already has that form in memory and uses that copy instead of reloading the form.
2. Forms already cached in the form cache on the local disk drive.
3. Forms published in the folder that is currently selected.
4. Forms in the Personal Forms Library.
5. Forms in the Organizational Forms Library.
6. Standard Outlook forms, such as Note, Post, and Contact, in the Application Forms Library.

Note

Before looking for a custom form, Outlook determines whether the message class of any form region matches the message class of the item being loaded. If there is a match, Outlook loads the form region. By default, Outlook also loads any form region that has a message class derived from the message class of the item, unless the exactMessageClass element of the form region is set to True. After loading the appropriate form regions, Outlook proceeds to look for forms in the order specified above. However, if any of the loaded form regions is a replacement or replace-all form region that has the loadLegacyForm element set to False, Outlook will not continue to look for and load any form that contains custom form pages. For more information on the exactMessageClass and loadLegacyForm elements, see the 2007 Microsoft Office System XML Schemas.

Because Outlook caches forms, you should avoid having more than one form with the same name or publishing the same form to more than one forms library. Forms used in a folder-based solution should be published only in the folder. If you are developing a solution based on mail message forms, you can temporarily publish the forms in your Personal Forms Library. After a form is finalized, you should publish it to the Organizational Forms Library on the Microsoft Exchange Server. You should then delete the form from your Personal Forms Library after making a backup copy. If you need to publish a form in more than one location, ensure that you keep all forms libraries up to date with the current version of the form.

Notes
It is not possible to disable the forms cache. If you set the size of the form cache to 0, Outlook cannot open any custom forms. If you are using an earlier version of Outlook, you can find more about forms cache in Description of Outlook 2002 forms cache and How to use the Outlook 2003 forms cache.