CA1862 suggestion changing semantics of the comparison.

[buyaa-n]

CA1862: Prefer using 'StringComparer'/'StringComparison' to perform case-insensitive string comparisons suggestion changing semantics of the comparison

Warning: Prefer using 'string.Equals(string, StringComparison)' to perform a case-insensitive comparison

using System;

// These are both lower case Greek Sigmas
string a = "σ";
string b = "ς";

// Prints True
Console.WriteLine(a.Equals(b, StringComparison.InvariantCultureIgnoreCase));

string lowerA = a.ToLowerInvariant();
string lowerB = b.ToLowerInvariant();

// Prints False
Console.WriteLine(lowerA == lowerB);

Originally posted by @MichalStrehovsky in #89630 (comment):
This is changing semantics of the comparison. Do we really have an analyzer that suggests people blindly do this?

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Issue Details

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          This is changing semantics of the comparison. Do we really have an analyzer that suggests people blindly do this?

using System;

// These are both lower case Greek Sigmas
string a = "σ";
string b = "ς";

// Prints True
Console.WriteLine(a.Equals(b, StringComparison.InvariantCultureIgnoreCase));

string lowerA = a.ToLowerInvariant();
string lowerB = b.ToLowerInvariant();

// Prints False
Console.WriteLine(lowerA == lowerB);

Originally posted by @MichalStrehovsky in #89630 (comment)

Author:	buyaa-n
Assignees:	-
Labels:	area-System.Globalization, untriaged, needs-area-label
Milestone:	-

[buyaa-n]

Created the issue in runtime in order to discuss the appropriate fix for this issue @carlossanlop @tarekgh PTAL

[tarekgh]

Using σ and ς is very-special case in the Unicode. If anyone is doing ToLower for comparisons in general will already be broken. For example, encountering Σ and using ToLower, will hold Σ == σ and Σ != ς. which would be wrong result. Actually, in general we recommend users to use ToUpper for compare casing because of such issues. Although, it is correct that Equals(b, StringComparison.InvariantCultureIgnoreCase) can have semantic change in this case, but it is better to have the analyzer flag this use cases of the ToLower anyway.

I prefer to keep the analyzer and we add something to the analyzer message clarify exceptional cases can hold different results. At least get users to think about it.

[MichalStrehovsky]

Using σ and ς is very-special case in the Unicode. If anyone is doing ToLower for comparisons in general will already be broken. For example, encountering Σ and using ToLower, will hold Σ == σ and Σ != ς. which would be wrong result.

Yep, this is true - but for the code that I pointed out, this a behavior of the CoreCLR implementation of reflection (that is written in C++) that NativeAOT reflection (written in C#) is simply matching. "Fixing" it on NativeAOT would introduce a behavioral difference between the reflection stacks. Whenever this analyzer kicks in, people need to be aware it can change behavior. It is very subtle.

[tarekgh]

Whenever this analyzer kicks in, people need to be aware it can change behavior. It is very subtle.

That is what I suggested to address that we add something to the analyzer message clarify exceptional cases can hold different results. At least get users to think about it.