[Analyzer] string.ToLower() == otherString.ToLower()

[stephentoub]

The right way to compare two strings in a case-insensitive manner is via a StringComparer/Comparison that specifies case-insensitivity. But there's are many cases where code is using ToLower() on each string and then comparing those for equality, e.g.

• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToUpper%5C%28&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToUpper%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23

We should write an analyzer that flags such uses and recommends fixing them to use Equals with the appropriate comparison.

This analyzer should cover ToUpper too even if the usage is low for that.

Such analyzers will help avoid the allocations caused by ToLower/ToUpper calls.

Performance rules Category
Severity = suggestion

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[MichalPetryka]

Should the invariant variants be also changed?

[stephentoub]

The intent is all variations of this that make sense. That includes ToLower/UpperInvariant.

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

Issue Details

---

The right way to compare two strings in a case-insensitive manner is via a StringComparer/Comparison that specifies case-insensitivity. But there's are many cases where code is using ToLower() on each string and then comparing those for equality, e.g.

• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToUpper%5C%28&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToUpper%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23

We should write an analyzer that flags such uses and recommends fixing them to use Equals with the appropriate comparison.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Runtime, code-analyzer, code-fixer
Milestone:	8.0.0

Tagging subscribers to this area: @dotnet/area-system-globalization
See info in area-owners.md if you want to be subscribed.

Issue Details

---

The right way to compare two strings in a case-insensitive manner is via a StringComparer/Comparison that specifies case-insensitivity. But there's are many cases where code is using ToLower() on each string and then comparing those for equality, e.g.

• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%20%3D%3D%20%5Cw%2B%5C.ToUpper%5C%28&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToLower%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToLower%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23
• https://grep.app/search?q=%5C.ToUpper%5C%28%5C%29%5C.Equals%5C%28%5Cw%2B%5C.ToUpper%5C%28%5C%29&regexp=true&case=true&filter[lang][0]=C%23

We should write an analyzer that flags such uses and recommends fixing them to use Equals with the appropriate comparison.

This analyzer should cover ToUpper too even if the usage is low for that.

Such analyzers will help avoid the allocations caused by ToLower/ToUpper calls.

Performance rules Category
Severity = suggestion

Author:	stephentoub
Assignees:	-
Labels:	api-suggestion, area-System.Globalization, code-analyzer, code-fixer
Milestone:	8.0.0