Question

I am designing a loosely-coupled structure. I want to call classes from different assemblies/namespaces via a code which is represented by a String. My design is, each of client’s business rules is on different assemblies and not dependent on each other (ONE client is to ONE DLL ratio) so that when I made an update on business rules of 1 client, it would not affect the others. My attention now is on using Factory Design and using Activator.CreateInstance() Method.

This is the project setup (2+n DLL’s)

 namespace Foundation; // where the interfaces/abstract resides
 namespace Factory; // has dependency on Foundation assembly
 namespace Client1; // client1's DLL, no dependency
 namespace Client2; // client2's DLL, no dependency
 The UI // only referenced to the Foundation and Factory not the Clients

The actual code

 namespace Foundation
 {
   public interface IBusinessRules
   {
     string GetBusinessRule();
  }
 }

 namespace Client1 //DLL for client 1
 {
   public class BusinessRules : Foundation.IBusinessRules
   {
    public string GetBusinessRule()
    {
        return "Client1 Business Rule";
    }
   }
}

namespace Client2 //DLL for client 2
{
   public class BusinessRules : Foundation.IBusinessRules
   {
     public string GetBusinessRule()
     {
        return "Client2 Business Rule";
     }
   }
}


namespace Factory
{
  public static class Invoker<T> where T: Foundation.IBusinessRules
  {
    public static T FetchInstance(string clientCode) 
    {
        return (T)Activator.CreateInstance(Type.GetType(clientCode));   
    }
  }
}


 //sample implementation that generates unhandled Exception
 using Factory;
 using Foundation;
 static void Main(string[] args)
 {
      //the parameter is maintained in the database
       IBusinessRules objClient1 = Invoker<IBusinessRules>.FetchInstance("Client1"); 

       //should call Client1.BusinessRules method
        Console.WriteLine(objClient.GetBusinessRule());  
        Console.Read();

        objClient = Invoker<IBusinessRules>.FetchInstance("Client2");

        //should call Client2.BusinessRules method
        Console.WriteLine(objClient.GetBusinessRule()); 
        Console.Read();      
  }

Any idea why my sample doesn’t work? And any suggestion to improve the design?
Thanks in advance.

How about using

Expression.Lambda

anyone?

Answer 1

Thanks to your help guys i finally Got it! I just modify the Factory

 namespace Factory
 {
    public static class Invoker<T> where T : Foundation.IBusinessRules
    {
        public static T FetchInstance(string clientCode)
        {
           Type objType = Type.GetType(clientCode + ".BusinessRules," + clientCode);
           return (T)Activator.CreateInstance(objType);

    }
}

But I wonder about its effeciency (performance hit) because it uses Reflection..