How to use dynamic dispatch with inheritance without converting to the appropriate subtype by hand?

[Charles]

Hello again,

I have the following situation, with a myFoo variable of type Foo that is 
instantiated as one of the foo subtypes:
    
    
    type
        
        Foo = ref object of RootObj
          x: string = "I'm Foo"
        
        Bar1 = ref object of Foo
          y: string = "I'm Bar1"
        
        Bar2 = ref object of Foo
          z: string = "I'm Bar2"
        
        FooManager = ref object
          myFoo: Foo
      
      var manager: FooManager = FooManager(
        myFoo: Bar1()
      )
      
      method showContents(b: Bar1) =
        echo b.y
      
      method showContents(b: Bar2) =
        echo b.z
      
      showContents(manager.myFoo)
    
    
    Run

That doesn't work because myFoo is a Foo type, not a Bar1 or Bar2 type, so I 
have to convert it by hand like so:
    
    
    method showContents(f: Foo) =
      if f of Bar1:
        showContents(f.Bar1)
      elif f of Bar2:
        showContents(f.Bar2)
    
    
    Run

which defeats the point.

(I also tried using converters but couldn't make it work.)

Is there anyway to make this work without having to check the type of myFoo?