String sorting issue in C#

Question

I have List like this

    List<string> items = new List<string>();
    items.Add("-");
    items.Add(".");
    items.Add("a-");
    items.Add("a.");
    items.Add("a-a");
    items.Add("a.a");

    items.Sort();

    string output = string.Empty;
    foreach (string s in items)
    {
        output += s + Environment.NewLine;
    }

MessageBox.Show(output);

The output is coming back as

-
.
a-
a.
a.a
a-a

where as I am expecting the results as

-
.
a-
a.
a-a
a.a

Any idea why "a-a" is not coming before "a.a" where as "a-" comes before "a."

Answer 1

I suspect that in the last case "-" is treated in a different way due to culture-specific settings (perhaps as a "dash" as opposed to "minus" in the first strings). MSDN warns about this:

The comparison uses the current culture to obtain culture-specific information such as casing rules and the alphabetic order of individual characters. For example, a culture could specify that certain combinations of characters be treated as a single character, or uppercase and lowercase characters be compared in a particular way, or that the sorting order of a character depends on the characters that precede or follow it.

Also see in this MSDN page:

The .NET Framework uses three distinct ways of sorting: word sort, string sort, and ordinal sort. Word sort performs a culture-sensitive comparison of strings. Certain nonalphanumeric characters might have special weights assigned to them; for example, the hyphen ("-") might have a very small weight assigned to it so that "coop" and "co-op" appear next to each other in a sorted list. String sort is similar to word sort, except that there are no special cases; therefore, all nonalphanumeric symbols come before all alphanumeric characters. Ordinal sort compares strings based on the Unicode values of each element of the string.

So, hyphen gets a special treatment in the default sort mode in order to make the word sort more "natural".

You can get "normal" ordinal sort if you specifically turn it on:

     Console.WriteLine(string.Compare("a.", "a-"));                  //1
     Console.WriteLine(string.Compare("a.a", "a-a"));                //-1

     Console.WriteLine(string.Compare("a.", "a-", StringComparison.Ordinal));    //1
     Console.WriteLine(string.Compare("a.a", "a-a", StringComparison.Ordinal));  //1

To sort the original collection using ordinal comparison use:

     items.Sort(StringComparer.Ordinal);