Linq

 public List<Platform> SomeMethod()
        {
            List<Marketplace> list = _dbContext.Set<Marketplace>().ToList();
 
            var tree = GetTree(list, 0);
        }
 
        protected List<Tree> GetTree(List<Marketplace> list, int parent)
        {
            return list.Where(x => x.PlatformId == parent).Select(x => new Tree
            {
                Id = x.Id,
                Name = x.Name,
                List = x.PlatformId != x.Id ? GetTree(list, x.Id) : new List<Tree>()
            }).ToList();
        }