Impossible to unwrap COM instance from custom domain

Question

I'm trying to make a call inside a COM components from a custom domain. The problem is when I try to unwrap my ObjectHandle an Serialization exception is throw.

But if I use the current AppDomain to create the instance, it's working....

The exception message:

An unhandled exception of type 'System.Runtime.Serialization.SerializationException' occurred in ConsoleApp1.exe
Additional information: Type 'MyAddin.Main' in assembly 'MyAddin, Version=2019.0.1.5, Culture=neutral, PublicKeyToken=null' is not marked as serializable.

And my code:

using System;

namespace ConsoleApp1
{
    class Program
    {

        static void Main(string[] args)
        {
            string addinPath = "C:\\sources\\MyAddin\\bin\\x64\\Debug\\MyAddin.dll";
            string addinFolder = "C:\\sources\\MyAddin\\bin\\x64\\Debug\\";
            string addinConfigPath = "C:\\sources\\MyAddin\\bin\\x64\\Debug\\MyAddin.dll.config";

            System.AppDomainSetup setup = new System.AppDomainSetup();
            setup.ApplicationBase = addinFolder;
            setup.ConfigurationFile = addinConfigPath;
            setup.ApplicationName = "MyAddin.dll";

            string strClsid = "{2616ad89-f4d1-4dc7-9d9d-a5de101b9085}"; // CLSID of my COM addin

            System.AppDomain customDomain = System.AppDomain.CreateDomain(strClsid,  null,  setup);
            // The type of domain is System.Runtime.Remoting.Proxies.__TransparentProxy}

            // The type of custom domain is 
            System.Type addinComType = System.Type.GetTypeFromCLSID(System.Guid.Parse(strClsid));

            System.Runtime.Remoting.ObjectHandle addinInstanceObjectHandle = customDomain.CreateComInstanceFrom(addinPath, addinComType.FullName); 
            System.Object addinInstance = addinInstanceObjectHandle.Unwrap(); //Throw the Serialization exception when create COM instance from customDomain
            // But working if I did System.AppDomain.CurrentDomain.CreateComInstanceFrom

            System.Reflection.MethodBase myMethod = addinInstance.GetType().GetMethod("connectToEwAPI");
            System.Object[] parameters = { null };
            myMethod.Invoke(addinInstance, parameters);

        }
    }
}

So I do something stupid ? Do you know what I'm missing ?

Thanks in advance for your help

---

Edit

I have tried with more basic Assembly. So I have an C# assembly with just this code

namespace ClassLibrary2
{
    public class Class1
    {
        public Class1()
        {

        }
        public string MyMethod() => "OK";
    }
}

And my exe code is just

static void Main(string[] args)
{
    string basePath = @"C:\source\MyAddin\ConsoleApp1\ClassLibrary2\bin\Debug";
    string dllName = @"ClassLibrary2";
    string typeName = "ClassLibrary2.Class1";
    string dllFullpath = $"{basePath}\\{dllName}.dll";

    try
    {
        ObjectHandle objectHandle = AppDomain.CurrentDomain.CreateInstanceFrom(dllFullpath, typeName);
        Object addinObject = objectHandle.Unwrap();
        var myAddinMethod = addinObject.GetType().GetMethod("MyMethod");
        string result = myAddinMethod.Invoke(addinObject, null) as string; // Working


        AppDomainSetup setup = new AppDomainSetup()
        {
            ApplicationBase = basePath,
            ApplicationName = dllName,
            ConfigurationFile = dllName + ".dll.config",
            PrivateBinPath = basePath
        };

        AppDomain customDomain = AppDomain.CreateDomain("MyDomain", null, setup);
        ObjectHandle objectHandleFromCustomDomain = customDomain.CreateInstanceFrom(dllFullpath, typeName);
        Object addinObjectFromCustomDomain = objectHandleFromCustomDomain.Unwrap();  // Exception thrown
        var myAddinMethodFromCustomDomain = addinObjectFromCustomDomain.GetType().GetMethod("MyMethod");
        string resultFromCustomDomain = myAddinMethodFromCustomDomain.Invoke(myAddinMethodFromCustomDomain, null) as string;
    }
    catch(Exception e)
    {
        var t = e.Message; // Exception thrown: 'System.Runtime.Serialization.SerializationException' by objectHandleFromCustomDomain.Unwrap()
    }
}

So as you can see, it's working with the default domain but not with the custom one....

Do you have any idea ?

Answer 1

If you want to access an object across an AppDomain boundary, you need to do one of two things:

• Make the class Serializable, e.g. by adding the [Serializable] attribute. In this case, a copy of the object is serailized and passed across the AppDomain boundary. Any membes you call will be on the copy, and will not affect the original object.

• Make the class derive from MarshalByRefObject. In this case, a reference to the object is marshalled across the AppDomain boundary, and any members you call will affect the object in its original AppDomain.