Foo.cs

// data Foo = Bar Int | Baz String
public abstract class Foo
{
   public sealed class Bar : Foo
   {
       public int Value { get; set; }
   }
   public sealed class Baz : Foo
   {
       public string Value { get; set; }
   }

   public T Fold<T>(Func<Bar, T> barFunc, Func<Baz, T> bazFunc)
   {
       if (this is Bar)
       {
           return barFunc((Bar) this);
       }
       else if (this is Baz)
       {
           return bazFunc((Baz) this);
       }
       else
       {
           throw new Exception("Oops");
       }
   }
}