Why is this an issue?
Certain characters, once normalized to lowercase, cannot make a round trip. That is, they can not be converted from one locale to another and then accurately restored to their original characters.
It is therefore strongly recommended to normalize characters and strings to uppercase instead.
Noncompliant code example
Thread.CurrentThread.CurrentCulture = new CultureInfo("tr-TR");
var areStringEqual = "INTEGER".ToLower() == "integer"; // Noncompliant, the result is false as the ToLower will resolve to "ınteger"
var areCharEqual = char.ToLower('I') == 'i'; // Noncompliant, the result is false as the ToLower will resolve to "ı"
var incorrectRoundtrip = "İ".ToLowerInvariant().ToUpper() == "I".ToLowerInvariant().ToUpper(); // Noncompliant, because of the lower we lose the information about the correct uppercase character
Compliant solution
Thread.CurrentThread.CurrentCulture = new CultureInfo("tr-TR");
var areStringEqual = "ınteger".ToUpperInvariant() == "ıNTEGER";
var areCharEqual = char.ToUpperInvariant('ı') == 'ı';
var correctRoundtrip = "İ".ToUpperInvariant().ToLower() != "I".ToUpperInvariant().ToLower();