C# Excel Addin - Cross-domain singleton Exception

I am developing an excel addin and in this addin there are several AppDomains. I need to have access to some shared data across each AppDomain so I decided to use a cross-AppDomain singleton. I followed what was described in this thread :

http://www.dolittle.com/blogs/einar/archive/2007/05/18/cross-appdomain-singleton.aspx

Because this is an excel addin, I had to modify it a little when creating the AppDomain that contains the singleton so that the correct base directory is used when searching for assemblies. Below is my modified version:

public class CrossAppDomainSingleton<T> : MarshalByRefObject where T : new()
{
    private static readonly string AppDomainName = "Singleton AppDomain";
    private static T _instance;

    private static AppDomain GetAppDomain(string friendlyName)
    {
        IntPtr enumHandle = IntPtr.Zero;
        mscoree.CorRuntimeHostClass host = new mscoree.CorRuntimeHostClass();
        try
        {
            host.EnumDomains(out enumHandle);

            object domain = null;
            while (true)
            {
                host.NextDomain(enumHandle, out domain);
                if (domain == null)
                {
                    break;
                }
                AppDomain appDomain = (AppDomain)domain;
                if (appDomain.FriendlyName.Equals(friendlyName))
                {
                    return appDomain;
                }
            }
        }
        finally
        {
            host.CloseEnum(enumHandle);
            Marshal.ReleaseComObject(host);
            host = null;
        }
        return null;
    }


    public static T Instance
    {
        get
        {
            if (null == _instance)
            {
                AppDomain appDomain = GetAppDomain(AppDomainName);
                if (null == appDomain)
                {
                    string baseDir = AppDomain.CurrentDomain.BaseDirectory;
                    appDomain = AppDomain.CreateDomain(AppDomainName, null, baseDir, null, false);
                }
                Type type = typeof(T);
                T instance = (T)appDomain.GetData(type.FullName);
                if (null == instance)
                {
                    instance = (T)appDomain.CreateInstanceAndUnwrap(type.Assembly.FullName, type.FullName);
                    appDomain.SetData(type.FullName, instance);
                }
                _instance = instance;
            }

            return _instance;
        }
    }
}

Here is my implementation of the CrossAppDomainSingleton :

public class RealGlobal : CrossAppDomainSingleton<RealGlobal>
{
    //ExcelApp Value Shared
    private Microsoft.Office.Interop.Excel.Application s_excelApp = null;

    public Microsoft.Office.Interop.Excel.Application GetExcelApp()
    {
        return s_excelApp;
    }

    public void SetExcelApp(Microsoft.Office.Interop.Excel.Application app)
    {
        s_excelApp = app;
    }
}

Once I try to use either the get or set method (I tried a property also but got no further), I systematically get an exception:

Nom inconnu. (Exception from HRESULT: 0x80020006 (DISP_E_UNKNOWNNAME))

or in English: Unknown Name. (Exception from HRESULT: 0x80020006 (DISP_E_UNKNOWNNAME))

Marshalling works fine when I keep built-in types, but given that the object I want to access (Microsoft.Office.Interop.Excel.Application) is a COM object, I fear that this is the problem.

I'm very new to Remoting and Marshalling. Any ideas? Does it have something to do with the serialization of a COM object?

Many thanks in advance! Sean

You certainly should not be passing that Application object around, it will cause endless trouble.

I suggest you write a small helper that you can call from each AppDomain to get the right Application object. There is a small snag in doing this, since the usual CreateObject approach will not always get the Excel Application instance for the process you are in. Andrew Whitechapel has an explanation and the right code here: http://blogs.officezealot.com/whitechapel/archive/2005/04/10/4514.aspx.

Finally, you should take some care with locale issues when calling the Excel COM object in other language environments. Sometimes calls need to be localised, or you need to swith the thread's UI language. Some info here: http://msdn.microsoft.com/en-us/library/aa168494(v=office.11).aspx and some info on what they do in VSTO here: http://blogs.msdn.com/b/eric_carter/archive/2005/06/15/429515.aspx.