LINQ Distinct operator, ignore case?

Ash_5023 0
Viewed 46166

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?

distinct comparison linq string c#
4 Answers

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)

Marc_Gravell_233540

[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
}
Ash_50230

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();
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