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:

[ComVisibleAttribute(true)]
public interface IExposedMethods
{
    void LoadDocument(string path);
}

[ComVisibleAttribute(true)]
[ClassInterface(ClassInterfaceType.None)]
public class ExposedClass : IExposedMethods
{
    public void LoadDocument(string path)
    {
        Globals.ThisAddIn.Application.Workbooks.Open(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);
addIn.Object.LoadDocument(@"c:\path\test.xlsm");

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.