C# List comparison

Question

This prints false. why is that and how can i make it so that it prints true? my goal is to have a list of people and a temp list of people that the user can edit. at the end of the users edit i want to check if the temp list is any different from the original list, so i can check if the program should bother saving the newly edited list.

static void EqualLists()
    {
        List<Person> listA = new List<Person>();
        List<Person> listB = new List<Person>();
        Person a = new Person()
        {
            name = "John",
            age = 18
        };
        Person b = new Person()
        {
            name = "John",
            age = 18
        };
        listA.Add(a);
        listB.Add(b);

        if(listA == listB)
        {
            Console.WriteLine("true");
        }
        else
        {
            Console.WriteLine("false");
        }
    }

maybe duplicate this https://stackoverflow.com/a/14237111/4588756 — Hossein, Sep 20 '20 at 12:02
This question reveals a problem in your understanding of the difference between values and references Perhaps this could help [Value vs Reference types](http://www.albahari.com/valuevsreftypes.aspx) — Steve, Sep 20 '20 at 12:08

score 1 · Accepted Answer · answered Sep 20 '20 at 12:03

First, we need to define what equality means. You clearly mean it as "the lists have the same semantic contents", but:

if(listA == listB) is a reference comparison, which means "are the same actual list instance"
this can be tweaked by using listA.SequenceEqual(listB), but this then needs to know what equality means per item
by default, this is going to use EqualityComparer<T>.Default, which for reference types, defaults to "things are equal if they are the same object instance"
to fix that you'd need to override GetHashCode() and Equals(object) correctly (and ideally also implement IEquatable<T>)

A simpler approach might be:

var same = listA.Select(p => (p.age, p.name)).SequenceEqual(
    listB.Select(p => (p.age, p.name)));

score 0 · Answer 2 · answered Sep 20 '20 at 12:05

0

you try to compare the reference of two different objects. you must compere the contents of the two list. have a look at https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.sequenceequal?view=netcore-3.1

answered Sep 20 '20 at 12:05

user3090253

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C# List comparison

2 Answers2