C# Comparing two lists of lists of objects

Question

I don't know where I make a mistake, but i cannot compare two lists of lists of (for example) integers.

Simple example:

List<List<int>> A = new List<List<int>>();
A.Add(new List<int>(new int[] { 1 }));
A.Add(new List<int>(new int[] { 2 }));

List<List<int>> B = new List<List<int>>();
B.Add(new List<int>(new int[] { 1 }));

if (A.Contains(B[0])){
    Console.WriteLine("TRUE");
else{
    Console.WriteLine("FALSE");
}

returns False. What is the correct way to compare in this kind of situation? I tried also Intersect and Except with same result.

What are you trying to achieve with `A.Contains(B[0])`? Do you want this to tell you whether any of the lists in A have exactly the same contents as the list provided? Identical elements in the same order? Does order matter? What if the list in B is a subset of a list in A? Should that be true or false? — yoozer8, Feb 20 '14 at 20:32
It's false because it is not the same list reference (It compares the list directly, not only the elements in it). — Pierre-Luc Pineault, Feb 20 '14 at 20:32
The outer `List<>` is not relevant here, your core problem is comparing 2 `List`. And is `{1, 2}` equal to `{2, 1}` ? — H H, Feb 20 '14 at 20:32

Bobson · Accepted Answer · 2014-02-20T20:45:11.587

It might help to visualize this:

A:
   A[0]:
       A[0][0]: 1
   A[1]:
       A[1][0]: 2
B:
   B[0]:
       B[0][0]: 1

When you call A.Contains(), you're asking whether the thing you're testing is either A[0] or A[1]. Since you're passing it B, and B is neither of those two, it returns false. Even if you were to pass it B[0] or B[0][0], you'd still get false, because none of those are the same object as A[0] or A[1].

SequenceEqual() is the function which will test whether the contents of two List are the same. So you want to test whether either A[0] or A[1] is SequenceEqual to B[0] (i.e. either A[0][0] == B[0][0] or A[1][0] == B[0][0]).

The best way to do this is with the LINQ function .Any(). A.Any(a => a.SequenceEqual(B[0])) will test B[0] against everything in A. If you want to compare all B elements to all A elements, you'd need something more like A.Any(a => B.Any(b => a.SequenceEqual(b)). This can be translated as:

foreach (var a in A) // A[0], A[1], etc.
{ 
    foreach (var b in B) // B[0], B[1], etc.
    {
       // is our current a value matching the current b value?
       // i.e. A[0] is exactly the same as B[0] on the first pass
       if (a.SequenceEqual(b)) return true; 
    }
}
return false;

I suspect, that problem is caused by some kind of reference equality, but i hadn't got any idea how to solve it. Thanks for explanation. — Iver, Feb 20 '14 at 20:50

score 3 · Answer 2 · answered Feb 20 '14 at 20:31

3

you can iterate through each list in A and compare it to B[0] with SequenceEqual:

if (A.Any(list => list.SequenceEqual(B[0])))
{
    Console.WriteLine("TRUE");
}
else
{
     Console.WriteLine("FALSE");
}

will return true

answered Feb 20 '14 at 20:31

Jonesopolis

25,034
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L.B · Answer 3 · 2014-02-20T20:51:45.730

3

bool contains = A.Any(a => B.Any(b => b.OrderBy(x => x)
                                       .SequenceEqual(a.OrderBy(x => x))));

 [[1],[2]]    and [[3]]       => false
 [[1],[2]]    and [[1]]       => true
 [[2],[1]]    and [[1],[2]]   => true
 [[3],[4]]    and [[1],[2]]   => false
 [[1,2],[3]]  and [[2,1],[4]] => true

edited Feb 20 '14 at 20:51

answered Feb 20 '14 at 20:36

L.B

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@Jonesy Question is not clear. I assumed `[2,1]` is equal to `[1,2]`. Otherwise `SequenceEqual` would return false. – L.B Feb 20 '14 at 21:23

score 0 · Answer 4 · answered Feb 20 '14 at 20:32

0

For reference types other than string, operator == returns true if its two operands refer to the same object. That's clearly not the case here you are comparing one list reference to another list reference.

answered Feb 20 '14 at 20:32

Peter R

2,865
18
19

It actually (surprisingly?) does the same thing for `string` if it is passed as generic parameter. – Grozz Feb 20 '14 at 20:38
@Grozz - the `==` is applied to the `List<>`s, not to the `T`s – H H Feb 21 '14 at 18:14
@HenkHolterman I understand. I'm just saying that if you have generic `Some` and compare its parameter with `==` it will use referential equality both for `T <- List` and `T <- string` – Grozz Feb 21 '14 at 19:10

score 0 · Answer 5 · edited May 23 '17 at 12:16

You are comparing lists, which by default are compared for reference equality. For this particular case you can use SequenceEqual extension in the following way:

bool contains = A.Any(a => a.SequenceEqual(B[0]));
Console.WriteLine(contains);

This may or may not be efficient enough for your purposes :)

Another way to approach this is to write your own IEqualityComparer wrapper around SequenceEqual as described here: IEqualityComparer for SequenceEqual and then use it as parameter in your .Contains call.

C# Comparing two lists of lists of objects

5 Answers5