Order list by parent and child items

.net, c#, linq

Solution

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();

Problem

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?

Original source