Introduction

Document-Level customizations refer to providing managed extensions to a particular document. E.g. a Word or Excel document. These customizations do not have any impact on other documents or Office Application as a whole.

A document level customization in form of a managed assembly gets attached to the document. Whenever that document is opened, the customization is loaded and ready to respond to events.

The reference between the document and customization is established by adding a custom property to the document called _AssemblyLocation. This property stores the location of Deployment Manifest.

The main design considerations of Office Document-Level Customization Architecture are following:

·         Each customization is loaded in new application domain so as to isolate it from main app and other customization. Thus if the customization hangs whole app doesn’t crashes.

·         Security and Permission level checks performed on customizations.

·         Clean shutdown of application and loaded customizations

While the scope of Application-Level Add-ins is whole application, the Document-Level customizations have been designed to work with a single document. A single app domain is created and cross document communication is not allowed.

Architecture Components

Runtime Loader

The component is named VSTOEE.dll and is responsible for loading the correct version of VSTO runtime when a document-level customization is opened or add-in is started.

Solution Loader

It is named VSTOLoader.dll. This is responsible for loading the VSTO application. It also creates a new Application Domain for each add-in/customization and performs security checks on VSTO application.

Deployment Manifest and Application Manifest

The Deployment manifest contains details about the customization artifacts. Each deployment manifest points to the correct version of an application manifest which in turn points to assembly paths and entry points.

Customization Loading Process

1.       Office application checks the custom document properties to see whether there are managed code extensions associated with the document.

2.       It loads Runtime Loader which in turn loads Solution Loader.

3.       Solution Loader loads the .NET Runtime and managed portion of VSTO Runtime.

4.       Latest versions of Deployment and Application manifests are downloaded. Once security checks are passed, latest customization assemblies are downloaded.

5.       The Visual Studio Tools for Office runtime creates a new application domain and loads customization assembly into the application domain.

6.       The Visual Studio Tools for Office runtime calls the Startup event handler in customization assembly.