controllers

public class RegionController : ApiController
{
    public IEnumerable<Region> Get()
    {
        yield return new Region { Id = Guid.NewGuid(), Name = "Region1" };

        yield return new Region { Id = Guid.NewGuid(), Name = "Region2" };

        yield return new Region { Id = Guid.NewGuid(), Name = "Region3" };
    }
}

public class RegionV2Controller : ApiController
{
    public IEnumerable<RegionV2> Get()
    {
        yield return new RegionV2
        {
            Id = Guid.NewGuid(),
            Name = "Region1",
            Cities = new List<City> { GetCity("City1 of region1"), GetCity("City2 of region1"), GetCity("City3 of region1") }
        };

        yield return new RegionV2
        {
            Id = Guid.NewGuid(),
            Name = "Region2",
            Cities = new List<City> { GetCity("City1 of region2"), GetCity("City2 of region2"), GetCity("City3 of region2") },
            HasContragents = true
        };

        yield return new RegionV2
        {
            Id = Guid.NewGuid(),
            Name = "Region3",
            Cities = new List<City> { GetCity("City1 of region3"), GetCity("City2 of region3"), GetCity("City3 of region3") },
            HasContragents = true
        };
    }

    private City GetCity(string name)
    {
        return new City { Id = Guid.NewGuid(), Name = name };
    }
}