替换string.ToUpper（）与StringComparison或类似，完全保留行为

Question

Using left.ToUpper() == right.ToUpper() is not the best option to compare strings, at least because of Performance issues. I want to refactor (fully preserving behavior!) this code, to something efficient, but can't achieve full equivalency for the special case.

So, here is a simple test method:

[TestCase("Strasse", "Straße", "tr-TR")]
[TestCase("İ", "i", "tr-TR")]
public void UsingToUpper_AndCurrentCultureIgnoreCase_AreSame(string left, string right, string culture)
{
    // Arrange, Act
    Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo(culture);
    var toUpper = left.ToUpper() == right.ToUpper();
    var stringComparison = left.Equals(right, StringComparison.CurrentCultureIgnoreCase);

    // Assert
    Assert.AreEqual(toUpper, stringComparison);
}

I tried two options, StringComparison.CurrentCultureIgnoreCase and StringComparison.OrdinalIgnoreCase both of them fails (in different cases).

So, the question:

Is there a way to compare two strings, without changing case and fully preserve the behavior of ToUpper()?

Answer 1

You would have to write your own custom Comparison method I am afraid.

ToUpper is making use of the Unicode metadata. Every character (Unicode code point) has a case as well as case mapping to upper- and lowercase (and title case). .NET uses this information to convert a string to upper- or lowercase. You can find the very same information in the Unicode Character Database.

You can supply a culture to the ToUpper method, but this would not be your goal. You can write your own customCulture like defined in this answer: Create custom culture in ASP.NET

However, there would not be any similar behaviour to the ToUpper method as mentioned before it uses Unicode metadata. You cannot force string Equals to use Unicode Characters.