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VSTO(Visual Studio Tools for Office) Development tools that are part of Visual Studio for enhancing Office applications. Based on Microsoft's .NET environment, VSTO is used to create Office "add-ins" that perform functions such as automatically filling in fields in a Word or Excel document and e-mailing the files or uploading them to a SharePoint server. As of VSTO 2005, support for enhancing Outlook was added.
Prior to VSTO, macros and Visual Basic for Applications (VBA) were the common methods used to customize Office programs. Although they are still used, VSTO makes it easier to code many types of enhancements, which reside as separate DLLs in the user's computer. See .NET Framework, VBA, DLL and Visual Studio.