Question

I have factory that looks something like the following snippet. Foo is a wrapper class for Bar and in most cases (but not all), there is a 1:1 mapping. As a rule, Bar cannot know anything about Foo, yet Foo takes an instance of Bar. Is there a better/cleaner approach to doing this?

public Foo Make( Bar obj )
{
    if( obj is Bar1 )
        return new Foo1( obj as Bar1 );
    if( obj is Bar2 )
        return new Foo2( obj as Bar2 );
    if( obj is Bar3 )
        return new Foo3( obj as Bar3 );
    if( obj is Bar4 )
        return new Foo3( obj as Bar4 ); // same wrapper as Bar3
    throw new ArgumentException();
}

At first glance, this question might look like a duplicate (maybe it is), but I haven’t seen one exactly like it. Here is one that is close, but not quite:

Factory based on Typeof or is a

Answer 1

I’m not sure what you actually want to achieve. I would probably try to make it more generic.

You could use attributes on Foo, which Bar it supports, then you create a list in a initializing phase. We are doing quite a lot of stuff like this, it make adding and connecting new classes very easy.

private Dictionary<Type, Type> fooOfBar = new Dictionary<Type, Type>();
public initialize()
{
  // you could scan all types in the assembly of a certain base class
  // (fooType) and read the attribute

  fooOfBar.Add(attribute.BarType, fooType);
}

public Foo Make( Bar obj )
{
  return (Foo)Activator.CreateInstance(fooOfBar(obj.GetType()), obj);
}