C＃比较对象列表的两个列表

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.

Answer 1

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

Answer 2

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

Answer 3

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] (ie 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;

Answer 4

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.

Answer 5

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.