Question

I’m attempting to make a generic routine within my program that will instantiate objects for me. The routines actually responsible for calling new Object() are located within a different assembly in a factory that must be instantiated.

namespace ExternalLib
{
    class Object1Factory
    {
        public Object1 getObject1()
            //...
    }
    class Object2Factory
        // Same implementation as Object1Factory
}

I am attempting to provide a routine with my application that will allow me to call the appropriate routine to “new up” the object for me.

namespace MyApp
{
    class Program
    {
        static void Main()
            //...
        static Object getNewObject(string typeName)
        {
            //This is where I have problems
        }
    }
}

I have access to the source code of both the external library as well as my program, although an entire re-write of the architecture is too costly.

What I’ve tried:
My initial idea was to use custom attributes on the factory and/or factory routine and use reflection to grab the method, then call the method. I was originally using the signature private static T getNew<T>() and attempting to use a switch statement on the type parameter, but realised getNewObject(string typeName) would be easier. Factories have the same constructor signatures, but have no inheritance link.

In summation: Routine to create objects is in factory in another assembly, how do I automatically call those routines to get objects based on type?

Answer 1

If you want to continue down this path – can you put all of the factory methods in a single factory class called something like ‘ObjectFactory’?

Type t = typeof(OtherAssembly.ObjectFactory);

MethodInfo m = t.GetMethods().Where(a => a.ReturnType.Name == typeName).FirstOrDefault();

return m.Invoke(null, new object[] { /* PARAMETERS */ });

EDIT – response to James’s comment on question stating he’d prefer to keep factory classes separate:

(Using Martin’s idea below and relies on all Factory classes being names “[Type]Factory”)

(Also has non-static methods on factory class)

Type t = Type.GetType(String.Format("OtherAssembly.{0}Factory", typeName));

var myFactory = Activator.CreateInstance( t );

MethodInfo m = t.GetMethods().Where(a => a.ReturnType.Name == typeName).FirstOrDefault();

return m.Invoke(myFactory, new object[] { /* PARAMETERS */ });