When developing InfoPath solutions with a task pane, it is possible to call into form's code (business logic) via the window.external.Window.XDocument.Extension mechanism. For example, if you have a public method GetUserName() in your form code, you can call it as window.external.Window.XDocument.Extension.GetUserName().

What if you want to build a reusable library that can be exposed to the taskpane? It turns out that it's pretty easy to do:

1.       Create a regular C# library project, implement the OM as you see fit (make sure the assembly has a strong name)

2.       In project Assembly properties, mark the assembly as COM-visible

3.       Reference the library in your InfoPath project the usual way

4.       Create an OM member and expose it as a public property

5.       You can now call methods via the new OM property

The key here is step #2, marking the assembly as COM-visible. This allows Javascript to access its members and properties.

For example, if your library is called MyObjectModel and it has a method GetData(), you can expose and call it as follows:

In your FormCode.cs:

// Initialize it when appropriate, probably in OnLoad()
private MyObjectModel _objectModel;

In your task pane code:

If you form needs to run in the Forms Server this becomes a little trickier because Forms Server business logic does not support member variables. Instead you have to store all state in the FormState object. Because of that you have to make your OM class serializable by adding [System.SerializableAttribute()] to the class definition.

Your public property then becomes this:

This assumes you have initialized FormState["ObjectModel"] to an instance of your OM class, for example, during OnLoad().

Important: if you are using InfoPath 2007 OM, you need to mark both your business logic and the OM assemblies with the [assembly: AllowPartiallyTrustedCallers()]attribute.