LINQ Distinct operator, ignore case?

Question

Given the following simple example:

    List<string> list = new List<string>() { "One", "Two", "Three", "three", "Four", "Five" };

    CaseInsensitiveComparer ignoreCaseComparer = new CaseInsensitiveComparer();

    var distinctList = list.Distinct(ignoreCaseComparer as IEqualityComparer<string>).ToList();

It appears the CaseInsensitiveComparer is not actually being used to do a case-insensitive comparison.

In other words distinctList contains the same number of items as list. Instead I would expect, for example, "Three" and "three" be considered equal.

Am I missing something or is this an issue with the Distinct operator?

Marc Gravell · Accepted Answer · 2009-01-12T07:38:04.297

282

StringComparer does what you need:

List<string> list = new List<string>() {
    "One", "Two", "Three", "three", "Four", "Five" };

var distinctList = list.Distinct(
    StringComparer.CurrentCultureIgnoreCase).ToList();

(or invariant / ordinal / etc depending on the data you are comparing)

edited Jan 12 '09 at 07:38

answered Nov 12 '08 at 06:41

Marc Gravell

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score 5 · Answer 2 · edited Oct 23 '12 at 13:20

5

[See Marc Gravells answer if you want the most concise approach]

After some investigation and good feedback from Bradley Grainger I've implemented the following IEqualityComparer. It suports a case insensitive Distinct() statement (just pass an instance of this to the Distinct operator) :

class IgnoreCaseComparer : IEqualityComparer<string>
{
    public CaseInsensitiveComparer myComparer;

    public IgnoreCaseComparer()
    {
        myComparer = CaseInsensitiveComparer.DefaultInvariant;
    }

    public IgnoreCaseComparer(CultureInfo myCulture)
    {
        myComparer = new CaseInsensitiveComparer(myCulture);
    }

    #region IEqualityComparer<string> Members

    public bool Equals(string x, string y)
    {
        if (myComparer.Compare(x, y) == 0)
        {
            return true;
        }
        else
        {
            return false;
        }
    }

    public int GetHashCode(string obj)
    {
        return obj.ToLower().GetHashCode();
    }

    #endregion
}

edited Oct 23 '12 at 13:20

Dave New

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answered Nov 12 '08 at 06:42

Ash

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6

You simply don't need this. See my reply. – Marc Gravell Nov 12 '08 at 06:43
2

Yes, your reply arrived just as I was clicking "Post Your Answer". – Ash Nov 12 '08 at 06:48
They were certainly with <20 seconds of each other, I recall. Still, implementing something like IEqualityComparer is still a useful exercise, if only for understanding how it works... – Marc Gravell Nov 12 '08 at 06:50
Thanks again, I'll let this this answer live then, unless anyone strongly objects. – Ash Nov 12 '08 at 06:54
This sample fails when initialized for the tr-TR culture if the current culture is en-US, because GetHashCode will report different values for I (U+0049) and ı (U+0131), whereas Equals will consider them equal. – Bradley Grainger Mar 26 '09 at 23:34

score 3 · Answer 3 · edited Apr 28 '17 at 15:54

3

 ## Distinct Operator( Ignoring Case) ##
  string[] countries = {"USA","usa","INDIA","UK","UK" };

  var result = countries.Distinct(StringComparer.OrdinalIgnoreCase);

  foreach (var v in result) 
  { 
  Console.WriteLine(v);
  }

OutPut will Be

   USA 
   INDIA
   UK

edited Apr 28 '17 at 15:54

Liam

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answered Apr 28 '17 at 11:11

Javed Ahmad

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Please avoid posting code snippets without explanation. Edit your answer and add a body to it. Thanks. – Clijsters Apr 28 '17 at 11:27

score 0 · Answer 4 · edited Dec 20 '11 at 16:52

0

Here is a far simpler version.

List<string> list = new List<string>() { "One", "Two", "Three", "three", "Four", "Five" };

var z = (from x in list select new { item = x.ToLower()}).Distinct();

z.Dump();

edited Dec 20 '11 at 16:52

Brandon

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answered Jan 13 '09 at 18:02

LINQ Distinct operator, ignore case?

4 Answers4

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