Order list by parent and child items

Question

I have a list of products that have to be ordered by parent then all children of that parent, then next parent, etc.

Product One
    Child One
    Child Two
Product Two
    Child One

These products are all in one table with a parent id field, the child products have a parent id but the parent items can have a null parent (indicating that product is a top level product)

I was thinking something like the following:

var list = GetProductList();
var newList = new List<ProductDTO>();

var parents = from p in list
    where p.Parent == null
    select p.Id;


foreach (var parent in parents)
{
    var tempList = new List<ProductDTO>();
    tempList.Add(list.FirstOrDefault(x => x.Id == parent));
    tempList.AddRange(list.Where(x => x.Parent == parent).OrderBy(x => x.Id));
    newList.AddRange(tempList);
}

Any suggestions on how I would do this a little cleaner?

Answer 1

you could try something like that. Assuming parent is a nullable:

var sorted = list.OrderBy(x => x.parent ?? x.id).ThenBy(x=>x.id);

if its a string:

            var sorted = list.OrderBy(x =>
            {
                if (x.parent == "null")
                    return x.id;
                else
                    return Convert.ToInt32(x.parent);
            }).ThenBy(x => x.id);

Answer 2

Given "Parent" is nullable property (assuming nullable int here). Following should give you parent-child related ordered list:

 public class ProductDTO
 {
     public int Id { get; set; }
     public string Name { get; set; }
     public int? Parent { get; set; }
 }

 var products = new List<ProductDTO>();
 products.Add(new ProductDTO() { Id = 1, Name = "Product One" });
 products.Add(new ProductDTO() { Id = 2, Name = "Product Two" });
 products.Add(new ProductDTO() { Id = 3, Name = "Product Three" });
 products.Add(new ProductDTO() { Id = 4, Name = "Child One", Parent=1 });
 products.Add(new ProductDTO() { Id = 5, Name = "Child Two", Parent = 2 });
 products.Add(new ProductDTO() { Id = 6, Name = "Child One", Parent = 1 });

var ordered = products
                .Where(p => p.Parent == null)
                .OrderBy(p=> p.Id)
                .Select(p => products
                    .Where(c => c.Parent == p.Id)
                    .OrderBy(c => c.Id))
                .ToList();