How to override general comparison for two strings in C#

Question

I've a question like the link below but much more complicated: Ignoring accented letters in string comparison

I have a dictionary and some values inside like:

{[Ministère de l'économie, 139]}
{[Ministère des finances, 114]}

and for inserting new elements into the dictionary, as the sames codes, I wrote this:

if (!dict.ContainsKey(str))
{  dict.Add(str, Convert.ToDouble(number)); }

But when I want to check the existence of this value : {[Ministère de l'economie, 139]} it returns it doesn't exist.

How can I implement this response to my code?

string.Compare(s1, s2, CultureInfo.CurrentCulture, CompareOptions.IgnoreNonSpace);

On the other way, as the title of my question, how can I overwrite the general (or the main) string comparison method in my application?

You need to implement an `IEqualityComparer` and supply it to the [constructor of your dictionary](https://msdn.microsoft.com/en-us/library/ms132072%28v=vs.110%29.aspx). In the comparer, you'll get the chance to define what is equal to what. — spender, Mar 03 '16 at 11:31
You cannot override string methods, except of redefine your string class. In you case, I'd recommand to use this method : http://stackoverflow.com/questions/249087/how-do-i-remove-diacritics-accents-from-a-string-in-net and then compare — cdie, Mar 03 '16 at 11:32

score 4 · Accepted Answer · edited May 23 '17 at 11:59

So, combining with the linked answer for removal of accents and diacritics, you can make an IEqualityComparer<string> to supply to your dictionary:

public class IgnoreAccentsAndDiacriticsComparer:IEqualityComparer<string>
{
    public bool Equals(string left, string right)
    {
        if(left == null && right == null){ return true; }
        if(left == null || right == null){ return false; }
        return string.Equals(RemoveDiacritics(left), RemoveDiacritics(right));
    }

    public int GetHashCode(string txt)
    {
        return RemoveDiacritics(txt).GetHashCode();
    }

    static string RemoveDiacritics(string text)
    {
        string formD = text.Normalize(NormalizationForm.FormD);
        StringBuilder sb = new StringBuilder();

        foreach (char ch in formD)
        {
            UnicodeCategory uc = CharUnicodeInfo.GetUnicodeCategory(ch);
            if (uc != UnicodeCategory.NonSpacingMark)
            {
                sb.Append(ch);
            }
        }

        return sb.ToString().Normalize(NormalizationForm.FormC);
    }
}

Now, construct your dictionary using an instance of this comparer:

var myDic = new Dictionary<string, int>(new IgnoreAccentsAndDiacriticsComparer())

and try adding something accented:

myDic["Aimée"] = 1;

...and read it back out, without the accent:

Console.WriteLine(myDic["Aimee"]); //ŵôõ!

score 1 · Answer 2 · edited May 23 '17 at 12:31

Starting from .NET 4.6 there are some interesting overloads of CompareInfo, so that the first solution suggested in https://stackoverflow.com/a/368850/613130 is usable in an IEqualityComparer<string>:

public class StringComparerIgnoreDiacritics : IEqualityComparer<string>
{
    public static readonly StringComparerIgnoreDiacritics CurrentCulture = new StringComparerIgnoreDiacritics(CultureInfo.CurrentCulture.CompareInfo);

    public readonly CompareInfo CompareInfo;

    public StringComparerIgnoreDiacritics(CompareInfo compareInfo)
    {
        CompareInfo = compareInfo;
    }

    #region IEqualityComparer<string> Members

    public bool Equals(string x, string y)
    {
        return CompareInfo.Compare(x, y, CompareOptions.IgnoreNonSpace) == 0;
    }

    public int GetHashCode(string obj)
    {
        return CompareInfo.GetHashCode(obj, CompareOptions.IgnoreNonSpace);
    }

    #endregion
}

and then

var myDic = new Dictionary<string, int>(var myDic = new Dictionary<string, int>(new IgnoreAccentsAndDiacriticsComparer()).CurrentCulture);

How to override general comparison for two strings in C#

2 Answers2