What is an ADDIN file format?

An .addin file is an XML-based definition file used by Microsoft Visual Studio to register custom add-ins. These add-ins are small software components that extend the functionality of Visual Studio by adding new features or automation tools. The .addin file acts as a manifest, instructing the IDE on how and when to load a particular add-in during startup.

Add-ins itself are small software applications that extend the functionality of bigger programs by getting to the primary programs. Addin files are stored in XML file format at the same location where the add-in project is created.

ADDIN File Format - More Information

ADDIN files are saved to disc in XML file format that is human readable. It can be opened in popular text editors including Notepad, Notepad++, Microsoft Visual Studio IDE, and many others. Microsoft has defined the XML manifest file of an Office Add-in that describes how an add-in should be activated after it is been installed and used with Office documents and applications.

See Also: How to build an Office COM add-in by using Visual C# .NET

Purpose of ADDIN Files

Visual Studio add-ins are designed to enhance developer productivity by automating tasks or integrating third-party tools directly into the IDE. The .addin file:

• Registers the add-in with the Add-In Manager.
• Specifies which version of Visual Studio should load the add-in.
• Allows reuse of the add-in across multiple projects without manual reconfiguration.

File Format and Structure

ADDIN files are saved in XML (Extensible Markup Language) format, which makes them human-readable and easy to edit with any text or code editor.

Sample .addin File

<Extensibility xmlns="http://schemas.microsoft.com/AutomationExtensibility">
  <HostApplication>
    <Name>Microsoft Visual Studio</Name>
    <Version>11.0</Version>
  </HostApplication>
</Extensibility>

Explanation:

• <Extensibility>: Root element for the add-in configuration.
• <HostApplication>: Defines the target application (in this case, Visual Studio).
• <Version>: Specifies which version of Visual Studio should load this add-in.

How to Open and Edit ADDIN Files

Because .addin files are plain text in XML format, they can be opened and edited using tools like:

• Notepad
• Notepad++
• Visual Studio
• VS Code
• Any XML Editor

Be cautious when editing these files-syntax errors can prevent the add-in from loading correctly.

Where Are ADDIN Files Located?

Typically, .addin files are placed in the following directories for Visual Studio to detect them:

• %USERPROFILE%\Documents\Visual Studio <version>\Addins
• Or the directory specified in the Tools > Options > Environment > Add-in/Macros Security settings

Visual Studio vs. Office Add-ins

It’s important not to confuse Visual Studio .addin files with Office Add-in manifests (also XML-based), which are used to extend applications like Word or Excel. Office add-ins are web-based and typically packaged differently (often using .manifest.xml and .vsix formats).

Common Use Cases

• Automating repetitive development tasks
• Creating custom tool windows or menu items
• Integrating third-party developer tools directly into Visual Studio

Security Considerations

Because add-ins can execute arbitrary code within the IDE, it’s important to:

• Only use .addin files from trusted sources
• Validate XML structure to avoid misconfigurations
• Use code signing where applicable for production deployments

Related File Formats

Note: .addin files are considered legacy in recent Visual Studio versions. Newer extensions use the .vsix format with a more advanced deployment model.

Troubleshooting .addin Files

If your add-in isn’t appearing:

• Verify the XML structure is valid.
• Ensure the .addin file is in the correct directory.
• Check Visual Studio’s Add-In Manager or Extensions window.
• Test compatibility with your current version of Visual Studio.

References

• Office Add-ins XML manifest