Exception unhandled when event is raised in dll (accessed using app domain)

Question

My project contains

windows form application(WindowsFormsApplication1)
dll (ClassLibrary1).
I am trying to implement some logic in my windows form application when an event is raised in my DLL. I am accessing DLL methods through AppDomain.

Below is the code in windows form Form1.cs
using System;
// more usings ...
using ClassLibrary1;
using System.Reflection;

namespace WindowsFormsApplication1  
{  
    public partial class Form1 : Form  
    {  
        public Form1()  
        {  
            InitializeComponent();  
            trial obj = new trial();  
            obj.trial1();  
         }  
      }  
  
    class trial   
    {  
        void HandleEvent(object sender, EventArgs e)  
        {  
            Console.WriteLine("HandleEvent called");  
        }  
  
         AppDomain ad = AppDomain.CreateDomain("Test");  
         string DLL = @"C:\..\ClassLibrary1.dll";  
              
           public void trial1()  
            {  
                 trial pro = new trial();MethodInfo method = typeof(trial).GetMethod("HandleEvent", BindingFlags.NonPublic | BindingFlags.Instance);  
  
                // Subscribe to the event  
                EventInfo eventInfo = typeof(ClassLibrary1.Class1).GetEvent("TestEvent");  
                Type type = eventInfo.EventHandlerType;  
                Delegate handler = Delegate.CreateDelegate(type, pro, method);  
                // method call 1  
                Loader.Call(ad, DLL,"ClassLibrary1.Class1","RaiseEvent",DateTime.Now.ToShortDateString());  
            }  
    }  
  
    public class Loader : MarshalByRefObject  
    {  
        object CallInternal(string dll, string typename, string method, object[] parameters)  
        {  
            Assembly a = Assembly.LoadFile(dll);  
            object o = a.CreateInstance(typename);  
            Type t = o.GetType();  
            MethodInfo m = t.GetMethod(method);  
            return m.Invoke(o, parameters);  
        }  
  
        public static object Call(AppDomain domain, string dll, string typename, string method, params object[] parameters)  
        {  
            Loader ld = (Loader)domain.CreateInstanceAndUnwrap(Assembly.GetExecutingAssembly().FullName, typeof(Loader).FullName);  
            object result = ld.CallInternal(dll, typename, method, parameters);  
            AppDomain.Unload(domain);  
            return result;  
        }  
    }  
}  

Below is the code in ClassLibrary1.dll
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ClassLibrary1  
{  
    public class Class1  
    {  
        public event EventHandler TestEvent;  
  
        public int RaiseEvent(string msg)  
        {  
            try  
            {  
                TestEvent(this, EventArgs.Empty);  
            }  
            catch (Exception ex)  
            {  
                Console.WriteLine("the exception is: " + ex.ToString());  
                if (ex.InnerException != null)  
                {  
                    Console.WriteLine("the inner exception is: " + ex.InnerException.Message.ToString());  
                }  
            }  
            return 2;  
         }  
    }  
}  

It throws an exception when it runs TestEvent(this, EventArgs.Empty); The exception is 'Object reference not set to an instance of an object.'
There are no issues when accessing the same event using a console application program. The code in the console application program are as follows.
using System;
using System.Reflection;

class Test  
{  
    void HandleEvent(object sender, EventArgs e)  
    {  
        Console.WriteLine("HandleEvent called");  
    }  
  
    static void Main()  
    {  
        Test test = new Test();  
        ClassLibrary1.Class1 publisher = new ClassLibrary1.Class1();  
        MethodInfo method = typeof(Test).GetMethod("HandleEvent", BindingFlags.NonPublic | BindingFlags.Instance);  
  
        // Subscribe to the event  
        EventInfo eventInfo = typeof(ClassLibrary1.Class1).GetEvent("TestEvent");  
        Type type = eventInfo.EventHandlerType;  
        Delegate handler = Delegate.CreateDelegate(type, test, method);  
  
        eventInfo.AddEventHandler(publisher, handler);  
        publisher.RaiseEvent("hello");  
    }  
}  

Any help will be very much appreciated. Thank you.

Answer 1

@Crystal98 , Welcome to Microsoft Q&A, based on my test, I get the same error with you.

I find that you used another method to call the method in the winform app. However, I noted that you only called the RaiseEvent method and you didn't add the event handler.

I make some changes and you could have a look.

public void trial1()  
        {  
            trial pro = new trial();   
            MethodInfo method = typeof(trial).GetMethod("HandleEvent", BindingFlags.NonPublic | BindingFlags.Instance);  
            Class1 publisher = new Class1();               //->here add it  
            // Subscribe to the event  
            EventInfo eventInfo = typeof(ClassLibrary1.Class1).GetEvent("TestEvent");  
            Type type = eventInfo.EventHandlerType;  
            Delegate handler = Delegate.CreateDelegate(type, pro, method);  
            eventInfo.AddEventHandler(publisher, handler);    //->here add it  
            publisher.RaiseEvent("Hello");                                  //->here add it  
          
        }  

Hope my code could help you.

Best Regards,
Jack

---

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Answer 2

It depends on what do you what to achieve and how to distribute the objects between domains.

For example, try this:

[Serializable]  
class trial  
{  
    void HandleEvent( object sender, EventArgs e )  
    {  
        Console.WriteLine( "HandleEvent called" );  
    }  
  
    AppDomain ad = AppDomain.CreateDomain( "Test" );  
    string DLL = @"....\ClassLibrary1.dll";  
  
    public void trial1( )  
    {  
        Loader.Call( ad, DLL, "ClassLibrary1.Class1", "RaiseEvent", HandleEvent, DateTime.Now.ToShortDateString( ) );  
    }  
}  
  
public class Loader : MarshalByRefObject  
{  
    object CallInternal( string dll, string typename, string method, EventHandler handler, object[] parameters )  
    {  
        Assembly a = Assembly.LoadFile( dll );  
        object o = a.CreateInstance( typename );  
        Type t = o.GetType( );  
  
        // Subscribe to the event  
        EventInfo eventInfo = t.GetEvent( "TestEvent" );  
        eventInfo.AddEventHandler( o, handler );  
  
        MethodInfo m = t.GetMethod( method );  
        return m.Invoke( o, parameters );  
    }  
  
    public static object Call( AppDomain domain, string dll, string typename, string method, EventHandler handler, params object[] parameters )  
    {  
        Loader ld = (Loader)domain.CreateInstanceAndUnwrap( Assembly.GetExecutingAssembly( ).FullName, typeof( Loader ).FullName );  
        object result = ld.CallInternal( dll, typename, method, handler, parameters );  
        AppDomain.Unload( domain );  
        return result;  
    }  
}