Question

I’m trying to write a C# function and make it accept any type parameter. I want to do it with generic list, but for some reason I can’t get it working. How to do it? Are there other ways?

public class City
{
    public int Id;
    public int? ParentId;
    public string CityName;
}

public class ProductCategory
{
    public int Id;
    public int? ParentId;
    public string Category;
    public int Price;
}

public class Test
{

        public void ReSortList<T>(IEnumerable<T> sources, ref IEnumerable<T> returns, int parentId)
        {
           //how to do like this:
           /*
            var parents = from source in sources where source.ParentId == parentId && source.ParentId.HasValue select source;

            foreach (T child in parents)
            {
                returns.Add(child);
                ReSortList(sources, ref returns, child.Id);
            }
           */
        }

        public void Test()
        {
            IList<City> city = new List<City>();

            city.Add(new City() { Id = 1, ParentId = 0, CityName = "China" });
            city.Add(new City() { Id = 2, ParentId = null, CityName = "America" });
            city.Add(new City() { Id = 3, ParentId = 1, CityName = "Guangdong" });

            IList<City> results = new List<City>();
            ReSortList<City>(city, ref results, 0);  //error
        }
}

Answer 1

If you only have ProductCategory and City objects (distinctly unrelated objects), just create two methods. Otherwise give both objects a common interface like IHasParentID so that you can expose the common functionality.