Share data between VSTO and Excel DNA App domains

Is there a way to share data between a VSTO add in and an Excel DNA add in? Or can the Excel DNA add in be loaded into the VSTO's app domain?

The Excel-DNA add-in will always be in a separate AppDomain. You might try to pass an object via the AddIn's Object property. Any MarshalByRef object works across AppDomains, but you need make the initial link...

Finally I ended up with this approach:

1) Put the sharable objects in a class lib and mark them COM visible:

namespace SharedLib
{
    using System.Runtime.InteropServices;

    [ComVisible(true)]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface ISharedObject
    {
        int GetVSTOInstanceNumber();
    }

    [ComVisible(true)]
    [ClassInterface(ClassInterfaceType.None)]
    public class SharedObject : ISharedObject
    {
        private readonly int seed;

        private int serial;

        public SharedObject(int seed)
        {
            this.seed = seed;
        }

        public int GetVSTOInstanceNumber()
        {
            return ++serial + this.seed;
        }
    }
}

 2) In VSTO create an instance of the shareable object and expose it via RequestComAddInAutomationService:

namespace SampleExcelAddIn
{
    using SharedLib;

    public partial class ThisAddIn
    {
        private const int Seed = 10;

        private ISharedObject sharedObject;

        protected override object RequestComAddInAutomationService()
        {
            if (sharedObject == null)
            {
                sharedObject = new SharedObject(Seed);
            }

            return sharedObject;            
        }

... code removed for brevity ...

    }
}

3) Now consume the shared object in the Excel DNA class lib:

namespace ExcelFunctions
{
    using ExcelDna.Integration;
    using Excel = Microsoft.Office.Interop.Excel;
    using SharedLib;

    public class ExcelFunctions
    {
        [ExcelFunction(Description = "Gets a serial number from the VSTO")]
        public static int GetVSTOInstanceNumber()
        {
            var application = (Excel.Application)ExcelDnaUtil.Application;
            object addinName = "SampleExcelAddIn";
            var addin = application.COMAddIns.Item(ref addinName);
            var utils = (ISharedObject)addin.Object;
            return utils.GetVSTOInstanceNumber();
        }
    }
}