How to Expose Methods in your VSTO Add-in

I have had this question a number of times and surprised myself when I did not see it in my blog. So here goes.

There are times when (like my Master Add-in Entry), you need to expose a set of methods or properties in your add-in so that other add-ins or applications are able to access them. Here is how it is done:

1) You create an interface and a class that uses that interface, like this:

public interface IExposedMethods
    void LoadDocument(string path);

public class ExposedClass : IExposedMethods
    public void LoadDocument(string path)


2) Next, you need to add the following code to your ThisAddin.cs:

private ExposedClass _exposedInstance;
protected override object RequestComAddInAutomationService()
    if (_exposedInstance == null)
        _exposedInstance = new ExposedClass();

    return _exposedInstance;

3) And finally, to call it and access this from another application instance, you do this:

Excel.Application xlApp = new Excel.Application();
xlApp.Visible = true;
object addinName = "MyAddinName";
Office.COMAddIn addIn = xlApp.COMAddIns.Item(ref addinName);

It is actually quite easy to implement. And the beauty here is that the ExposedClass you created can have all sorts of methods and properties that are exposed. And because the _exposedInstance is global to ThisAddIn (via Globals.ThisAddin._exposedInstance), you can access it anywhere. So it is a perfect way for one add-in to contact another or for a parent Windows Form application to reach into the Add-in and set properties and call methods and have it open documents and such for you.

5 thoughts on “How to Expose Methods in your VSTO Add-in

  1. Hello

    And how it will be the same thing but for a WorkBook?
    (VS2012 with an Office 2010 Workbook, then also InteropServices)

    • I am not sure I follow the question. Are you asking about a Workbook Project (Document-level solution)? If so, I do not believe this method will work. I am not sure how a document level solution would be exposed in the running instance since it is not really, at its base, an extensibility project.

      • Hello again

        Thanks for your answer

        I am just starting with VSTO, before I did few things with interop services.
        Just a WPF app over workbooks.
        So I was looking for a fast way (that does not exist)


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