What is wrong with my List<T>.Distinct()?

Question

I have a class MyItems that implements IEqualityComparer and overrides the following methods:

public bool Equals(MyItems item1, MyItems item2)
{
    return (item1.ID == item2.ID && item1.itemName.Equals(item2));
}
public int GetHashCode(MyItems item)
{
    return item.ID.GetHashCode() ^ item.itemName.GetHashCode();
}

First, why is GetHashCode necessary? I understand overriding the Equals method, however, the GetHashCode necessity has eluded me.

Second, this doesn't appear to be working. Is there something I'm doing wrong here? Where I don't understand the GetHashCode, that maybe where I am tripping up.

Answer 1

To answer your first question, just look here for more information.

To answer your second question: You forgot item2 should be item2.itemName

return (item1.ID == item2.ID && item1.itemName.Equals(item2.itemName));

Answer 2

The Distinct method works as follows:

1. Check if the two objects has the same hash code using GetHashCode.
2. If they do, now make sure they absolutely equals with Equals.

The GetHashCode is a first check for the more expensive check: Equals

Your Equals method has an error:

return (item1.ID == item2.ID && item1.itemName.Equals(item2));

Should be:

return (item1.ID == item2.ID && item1.itemName.Equals(item2.itemName));
//                                                         ^^^^^^^^^

Also, if the List or the array type you're using isn't of <MyItems> type you also need to override the Equals method.

Answer 3

If you want to compare objects you should override Equals(object obj) in their class.

Also, whenever you override Equals(object obj) it is good practice to override GetHashCode