c# - Intersect with a custom IEqualityComparer using Linq -

long story short: have 2 collections of objects. 1 contains values (let's call "good"), other default values (mr. "default"). want intersect of union between , default, , default. in other words: intersect(union(good, default), default). 1 might think resolves default, here gets tricky : use custom iequalitycomparer.

i got following classes :

class myclass {     public string mystring1;     public string mystring2;     public string mystring3; }  class myequalitycomparer : iequalitycomparer<myclass> {     public bool equals(myclass item1, myclass item2)     {         if(item1 == null && item2 == null)             return true;         else if((item1 != null && item2 == null) ||                 (item1 == null && item2 != null))             return false;          return item1.mystring1.equals(item2.mystring1) &&                item1.mystring2.equals(item2.mystring2);     }      public int gethashcode(myclass item)     {         return new { item.mystring1, item.mystring2 }.gethashcode();     } } 

here characteristic of collections , default collections :

default : it's large set, containing wanted { mystring1, mystring2 } pairs, mystring3 values are, can guess, default values.

good : it's smaller set, containing items in default set, mystring3 values. has { mystring1, mystring2 } outside of wanted set.

what want : take items in default, add other items in default that.

here is, think is, best try :

halfwantedresult = good.union(default, new myequalitycomparer()); wantedresult= halfwantedresult.intersect(good, new myequalitycomparer()); 

i taught should have worked, result { mystring1, mystring2 } pairs set, coming default set, have default value across. tried switching default , of last intersect, same result.

first of wrong:

public bool equals(myclass item1, myclass item2) {     return gethashcode(item1) == gethashcode(item2); } 

if hashcode's different sure corresponding 2 items different, if they're equal not guaranteed corresponding 2 items equal.

so correct equals implementation:

public bool equals(myclass item1, myclass item2) {     if(object.referenceequals(item1, item2))         return true;     if(item1 == null || item2 == null)         return false;     return item1.mystring1.equals(item2.mystring1) &&            item1.mystring2.equals(item2.mystring2); } 

as slacks suggested (anticipating me) code following:

var default = new list<myclass> {     new myclass{mystring1="a",mystring2="a",mystring3="-"},     new myclass{mystring1="b",mystring2="b",mystring3="-"},     new myclass{mystring1="x",mystring2="x",mystring3="-"},     new myclass{mystring1="y",mystring2="y",mystring3="-"},     new myclass{mystring1="z",mystring2="z",mystring3="-"},  }; var = new list<myclass> {     new myclass{mystring1="a",mystring2="a",mystring3="+"},     new myclass{mystring1="b",mystring2="b",mystring3="+"},     new myclass{mystring1="c",mystring2="c",mystring3="+"},     new myclass{mystring1="d",mystring2="d",mystring3="+"},     new myclass{mystring1="e",mystring2="e",mystring3="+"}, }; var wantedresult = good.intersect(default, new myequalitycomparer())                        .union(default, new myequalitycomparer());  // wantedresult: // a + // b b + // x x - // y y - // z z -