c# - Hashset not adding a duplicate, but returning true for Add() -

(edit) more info. first notice "new virtual". class inherits base class supposed generic parent-aware class can created icollection type. here's descriptor, basically:

public abstract class parentawarecollection<tobject, tparent, tinnerlist> :  icollection<tobject>, icollection, iparentprovider<tparent>     tobject : iparentprovider<tparent>     tinnerlist : icollection<tobject>, new() {         protected tinnerlist innerlist = new tinnerlist(); ... } 

this class:

 public class parentawarehashset<tobject, tparent> :     parentawarecollection<tobject, tparent, hashset<tobject>>, iset<tobject>,     icollection  tobject : iparentprovider<tparent>     {         public parentawarehashset(tparent parent)             : base(parent) 

iparentprovider requires "parentcollection" object present.

in debugging, overridden (void) add method in parentawarecollection never called. don't think "new" problem.

also, here's don't understand. here's descriptor actual hashset:

public class hashset<t> : iserializable, ideserializationcallback,  iset<t>, icollection<t>, ienumerable<t>, ienumerable { ...         public bool add(t item); } 

notice in implements icollection mine. led me believe ok do. unlike mine, framework hashset not use new descriptor add() method, that's required because icollection<t> implements void add(t item). perhaps omitted it?

original question:

i have collection class uses hashset<t> store objects. objects of type t override gethashcode(), in make sure required information present used produce hash code. not sure if important.

what happens when add(t) hashset, not add object, returns true. if debug , in immediate window try add again, returns false, it's supposed to. method looks this:

public new virtual bool add(tobject item) {     // must add parent first, since may used in hash code     // innerlist hashset<t>      if (innerlist.any(existing=>item.gethashcode()==existing.gethashcode())) {         return(false);     } else {         if (innerlist.add(item))         {             return(true);         } else {             return(false);         }     } } 

i added first condition make code work. works expected there. however, can't understand why have this, , can think of no reason, ever, why hashset return "true" add() without adding anything. if gethashcode override messed up, should either add , return true, or not , return false. thoughts?

here's observed while debugging:

breaking before innerlist.add():

?innerlist.count 1 ?item.gethashcode() -1629834529 ?innerlist.elementat(0).gethashcode() -1629834529 

step on innerlist.add(), returns true:

?innerlist.count 1 ?innerlist.add(item) false 

wtf? .net 4.0 framework.

your add code wrong:

public new virtual bool add(tobject item) {     // must add parent first, since may used in hash code     // innerlist hashset<t>      if (innerlist.any(existing=>item.gethashcode()==existing.gethashcode())) {         return(false);     } else {         if (innerlist.add(item))         {             return(true);         } else {             return(false);         }     } } 

two different items can have same hash code! function should this:

public new virtual bool add(tobject item) {     return innerlist.add(item); }