VSTO/COM add-ins Vs Office 365 web add-ins: Key differences to know

Microsoft Outlook is one of the most widely used email clients in the world. With its robust features and capabilities, it has become an essential tool for businesses and individuals alike. One of the key advantages of Outlook is its ability to integrate with various add-ins that can enhance its functionality and make it even more powerful.

Two popular types of add-ins for Outlook are VSTO (Visual Studio Tools for Office) add-ins and web add-ins. Both offer unique benefits and features, but there are some key differences between them that users should be aware of.

VSTO Outlook Add-Ins

VSTOs (well known as COM add-ins) are a set of tools that allow developers to create custom add-ins for Microsoft Office applications, including Outlook. VSTO add-ins are typically created using Visual Studio, which provides a rich development environment and a wide range of tools and resources. These add-ins are compatible with Microsoft Office 2010 and later versions, but cannot be used in the web version of Microsoft Outlook.

One of the main advantages of VSTO add-ins is that they are fully integrated with the Office suite. This means that they have access to all of the features and capabilities of the Office applications, including classic Outlook. VSTO add-ins can interact with the user interface, access data stored in Office documents, and even automate complex tasks.

Another advantage of VSTO add-ins is that they can be installed locally on a user’s machine. This means that they can be used even when the user is offline or disconnected from the internet. Additionally, because VSTO add-ins are installed locally, they can often provide better performance and reliability than web add-ins.

Web Add-Ins for Microsoft Outlook

Web add-ins for Microsoft Outlook are a newer technology that allows developers to create add-ins using web technologies such as HTML, CSS, and JavaScript. Unlike VSTO add-ins, web add-ins are hosted on a web server and accessed through a web browser. These add-ins are compatible with Office 2013 and subsequent versions and can also be used in Office Online and Office for Mac.

One of the main advantages of web add-ins is that they can be accessed from anywhere, on any device with an internet connection. This makes them ideal for users who need to access their email and other Office applications from multiple devices or locations.

Another advantage of web add-ins is that they can be updated and maintained more easily than VSTO add-ins. Because web add-ins are hosted on a web server, developers can update the add-in code without requiring users to install new software or updates.

However, there are also some limitations to web add-ins. For example, because they are accessed through a web browser, they may not be able to access all of the features and capabilities of Outlook. Additionally, because they rely on an internet connection, they may be slower or less reliable than locally installed VSTO add-ins. Also, since they depend on an Internet connection, they may be slower or less reliable than locally installed VSTO add-ons. One last minor feature is that these add-ons are very limited in available features and are not designed for bulk operations on Outlook items.


In conclusion, both VSTO Outlook add-ins and web add-ins offer unique benefits and capabilities for users and developers. VSTO add-ins provide full integration with the Office suite and can be installed locally for better performance and reliability. Web add-ins, on the other hand, offer the ability to access email and other Office applications from anywhere, on any device, and can be updated more easily.

Ultimately, the choice between VSTO and web add-ins will depend on the specific needs and requirements of the user or organization. Developers should carefully consider the advantages and limitations of each technology before deciding which approach to take. And of course such a choice arises only when the necessary functionality is available in both versions of the add-ins, which, unfortunately, is extremely rare. VSTO add-ins can do and automate everything in Microsoft Outlook, web add-ins can do only some things.

To learn more about the add-ins and its support read this article

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