How polymorphism work in D?

[OiseuKodeur]

I have this

```
import std.stdio : writeln;

abstract class Foo { }

class Bar : Foo
{
     float value;

     this(float t_value) { value = t_value; }
}

class Baz : Foo
{
     string name;

     this(string t_name) { name = t_name; }
}

void main()
{
     Foo foo = new Bar(10);

     if (/* typeof foo == Bar */)
         foo.value.writeln;
     else if (/* typeof foo == Baz */)
         foo.name.writeln;

     foo.writeln;
}
```

I don't understand how i can differentiate between Bar and Baz 
and to get their attributes