C# linq groupby returns incorrect groups -

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i trying understand how linq works. wrote test app , not working way expect to. following code, expecting see items "test1" , "test4" grouped together, not getting that. instead getting 4 separate groups. meaning 1 of items getting grouped together. can explain doing wrong? thanks.

public class linqtest {   public int x1;     public int x2;     public string x3;      public linqtest(int a, int b, string c)     {         x1 = a;         x2 = b;         x3 = c;      }      public bool equals(linqtest other)     {          if (referenceequals(null, other)) return false;         if (referenceequals(this, other)) return true;          return x1 == other.x1 &&                 x2 == other.x2;      }      public override bool equals(object obj)     {         if (referenceequals(null, obj)) return false;         if (referenceequals(this, obj)) return true;         if (obj.gettype() != typeof(linqtest)) return false;         return equals((linqtest)obj);     } } linqtest tc14 = new linqtest(1, 4, "test1"); inqtest tc15 = new linqtest(3, 5, "test2"); linqtest tc16 = new linqtest(3, 6, "test3"); linqtest tc16a = new linqtest(1, 4, "test4");  list<linqtest> tclistitems = new list<linqtest>(); tclistitems.add(tc14); tclistitems.add(tc15); tclistitems.add(tc16); tclistitems.add(tc16a);  ienumerable<igrouping<linqtest, linqtest>> tcgroup = tclistitems.groupby(c => c);

why tcgroup contain 4 groups? expecting 3 groups.

the error happens because override equals without overriding gethashcode. these 2 must overriden together, otherwise groupby not work.

add code class fix issue:

public override int gethashcode() {     // ignoring x3 equality, hash code must ignore     return 31*x1+x2; }

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