Substring comparer on Intersect

Question

I need to do an intersect between strings but comparing substrings:

public class MiNumeroEqualityComparer : IEqualityComparer<string> {
    public bool Equals(string x, string y) => x.Contains(y);
    public int GetHashCode(string obj) => obj.GetHashCode();
}

List<string> lst = new List<string> { "abcXdef", "abcXdef", "abcede", "aYcde" };
List<string> num = new List<string> { "X", "Y", "Z" };

var fin = lst.Intersect(num, new MiNumeroEqualityComparer());

I expect in fin: "abcXdef", "abcXdef", "aYcde"

But it's empty, why?

First I've tried substring with case insensitive with: (without success too)

public bool Equals(string x, string y) => x.IndexOf(y, StringComparison.InvariantCultureIgnoreCase) >= 0;

But empty too.

Your equality comparer is broken for a start. You seem to be trying to use it to make "abcXdef" equal "X" but you have many problems. For a start your equals is not reverable for example. So `Equals(x,y)` is not going to give the same result as `Equals(y,x)`. Also your getHashCode is wrong which is likely why nothing is happening. "abcXdef" has a different hashcode to "X" so there is no way they can be the same so it probably isn't even running your Equals methods... — Chris, Oct 11 '18 at 23:59
Thanks for the comment, I've tried GetHashCode return 1 for all, some strings are on the list but not all should be, I suppose that Linq internally has several optimizations and not all elements check the Equals function. — Enrique, Oct 12 '18 at 09:33
The point of my comment was not just to point out flaws in your equality implementation but also to question whether its the right way to do things. "x" and "abcXdef" are not equal in any practical sense so making an equality comparer that says they are is silly. given that for an equality comparer if a=b and b=c then a =c should be true with your logic that two strings are equal if one is a substring of another you would eventually end up having to prove all strings are equal to each other more or less. — Chris, Oct 12 '18 at 12:19
You're complete right @Chris, thanks! Now, I understand quite well. — Enrique, Oct 12 '18 at 14:21

Rufus L · Accepted Answer · 2018-10-11T23:48:27.877

You're doing an intersection between two lists, which will give you the common items between them. Since neither list contains an identical item, you are getting no results.

If you want to get all the items from lst that contain an item from num, then you can do something like the code below, which uses the string.Contains method to filter the items from lst:

var fin = lst.Where(item => num.Any(item.Contains));

Result:

{ "abcXdef", "abcXdef", "aYcde" }

Alternatively, if you do want to do a case-insensitive query, you can use the IndexOf method instead:

var fin = lst.Where(item => num.Any(n => 
    item.IndexOf(n, StringComparison.OrdinalIgnoreCase) >= 0));

If that's hard to understand (sometimes Linq is), the first code snippet above is a shorthand way of writing the following:

var fin = new List<string>();

foreach (var item in lst)
{
    foreach (var n in num)
    {
        if (item.Contains(n))
        {
            fin.Add(item);
            break;
        }
    }
}

Thanks a lot, I'm newbie in Linq questions, with foreach's it's quite easy to do for me, but I'd want to learn/understand Linq. — Enrique, Oct 12 '18 at 09:43

score 1 · Answer 2 · answered Oct 12 '18 at 00:08

Sure Rufus has solved your issue in the answer provided. But let me explain why your approach was not working.

The reason it is producing an empty result is because Equals(string x, string y) will never be called. It can infer the inequality from the GetHashCode method. If the hashes are the same, then it will call Equals. In other words, your logic in Equals will never be executed.

Here is some code so you can see what is going on.

class Program
{
    static void Main(string[] args)
    {
        // See I added an item at the end here to show when Equals is called
        List<string> lst = new List<string> { "abcXdef", "abcXdef", "abcede", "aYcde", "X" };
        List<string> num = new List<string> { "X", "Y", "Z" };

        var fin = lst.Intersect(num, new MiNumeroEqualityComparer()).ToList();
        Console.ReadLine();
    }
}

public class MiNumeroEqualityComparer : IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {
        Console.WriteLine("Equals called for {0} and {1}.", x, y);
        return x.Contains(y);
    }

    public int GetHashCode(string obj)
    {
        Console.WriteLine("GetHashCode alled for {0}.", obj);
        return obj.GetHashCode();
    }
}

If you run the above code, it will only call Equals for items which produce the same hash; so for "X" only.

See the output in this fiddle.

Thank you very much for the explanation, I understand now. I don't know [dotnetfiddle.net](https://dotnetfiddle.net) very useful, thanks! — Enrique, Oct 12 '18 at 10:05

score 0 · Answer 3 · answered Oct 11 '18 at 23:33

Intersect gets common elements from 2 collections. The Intersect method here is elegant. It can be used on many types of elements.

your result is empty because it is not a common value in the lists.

  List<string> lst = new List<string> { "abcXdef", "abcXdef", "abcede", "aYcde" };
            List<string> num = new List<string> { "X", "Y", "abcXdef", "Z", "aYcde" };

            var fin = lst.Intersect(num);

fin >> abcXdef,aYcde

Substring comparer on Intersect

3 Answers3