Creating and using a custom List<T> in C# -

i trying use customized list have added few additional tools. want apply list long list of customized classes have created. of classes have id number , of tools in list use id.

here portion of code trying use. hope understand problem.

namespace simple_point_of _sales_system {     public class mylist<t> : list<t>     {         internal int setid()         {             return this.max(n => n.id) + 1;         }         internal t find(int id)         {             return this.find(n => n.id == id);         }         internal t add(t n)         {             read();             add(n);             write();             return n;         }         internal void remove(int id)         {             read();             if (this.exists(t => t.id == id)) removeall(t => t.id == id);             else messagebox.show(gettype().name + " " + id + " not exist.", "missing item", messageboxbuttons.ok, messageboxicon.error);             write();         }         internal void edit(int id, t n)         {             read();             if (this.exists(t => t.id == id)) this[findindex(t => t.id == id)] = n;             else messagebox.show(gettype().name + " " + id + " not exist.", "missing item", messageboxbuttons.ok, messageboxicon.error);             write();         }         internal mylist<t> read()         {             clear();             streamreader sr = new streamreader(@"../../files/" + gettype().name + ".txt");             while (!sr.endofstream)                 add(new t().set(sr.readline()));             sr.close();             return this;         }         internal void write()         {             streamwriter sw = new streamwriter(@"../../files/" + gettype().name + ".txt");             foreach (t n in this)                 sw.writeline(n.tostring());             sw.close();         }     }      public class customer     {         public int id;         public string firstname;         public string lastname;     }      public class item     {         public int id { get; set; }         public string category { get; set; }         public string name { get; set; }         public double price { get; set; }     }      public class myclass     {         mylist<customer> customers = new mylist<customer>();         mylist<item> items = new mylist<item>();     } } 

i think custom list needs put on constraints on generic type allow that. update signature like

public class mylist<t> : list<t> t : iidentity { .... } 

here used iidentity interface defining id, class.

to update code this:

public interface iidentity {     int id { get; } }  public class customer : iidentity {     public int id { get; set;}     public string firstname;     public string lastname; }  public class item : iidentity {     public int id { get; set; }     public string category { get; set; }     public string name { get; set; }     public double price { get; set; } } 

i changed id in customer property instead of instance variable.