C#カスタムクラスの集合ソートの問題

 1 class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             var sl = GetS();
 6             sl.Sort(new Comparison<SItem>((x, y) =>
 7             {
 8                 return x.ComString(y);   //              
 9             }
10             ));
11            
12 
13             foreach (var item in sl)
14             {
15                 Console.WriteLine(item.Id+" "+item.Size+" "+item.SKU);
16             }
17         }
18 
19         public static List<SItem> GetS()
20         {
21             var temp = new List<SItem>();
22             temp.Add(new SItem { Id = "002", Size = 2, SKU = "Test" });
23             temp.Add(new SItem { Id = "001", Size = 2, SKU = "Test" });
24             temp.Add(new SItem { Id = "003", Size = 2, SKU = "Test" });
25             temp.Add(new SItem { Id = "002", Size = 2, SKU = "Test" });
26             return temp;
27         }   
28     }
29 
30 
31     class SItem
32     {
33         public string Id { get; set; }
34         public string SKU { get; set; }
35         public int? Size { get; set; }
36     }
37 
38     static class Linq
39     {
40         static public int ComString(this SItem s,SItem t)
41         {
42             var a = s.Id;
43             var b = t.Id;
44             if (a.Length!=b.Length)
45             {
46                 if (a.Length>b.Length)
47                 {
48                     return 1;
49                 }
50                 else
51                 {
52                     return -1;
53                 }
54             }
55             for (int i = 0; i < a.Length; i++)
56             {
57                 if (a[i]>b[i])
58                 {
59                     return 1;
60                 }
61                 if (a[i]<b[i])
62                 {
63                     return -1;
64                 }
65             }
66             return 0;
67         }
68     }