[Analyzer] Using StartsWith instead of IndexOf == 0

[stephentoub]

We should write an analyzer that flags uses of IndexOf that's comparing the resulting index to 0, e.g.
https://grep.app/search?q=%5C.IndexOf%5C%28.%2A%3F%5C%29%20%3D%3D%200&regexp=true&case=true&filter[lang][0]=C%23
Such use would be much better off with StartsWith, as IndexOf will search the entire string vs StartsWith which will only compare at the beginning.

Performance rules Category
Severity = suggestion

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Issue Details

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We should write an analyzer that flags uses of IndexOf that's comparing the resulting index to 0, e.g.
https://grep.app/search?q=%5C.IndexOf%5C%28.%2A%3F%5C%29%20%3D%3D%200&regexp=true&case=true&filter[lang][0]=C%23
Such use would be much better off with StartsWith, as IndexOf will search the entire string vs StartsWith which will only compare at the beginning.

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

[MichalPetryka]

Same with Contains and IndexOf != -1/IndexOf > -1/IndexOf >= 0.

[stephentoub]

Same with Contains and IndexOf != -1/IndexOf > -1/IndexOf >= 0.

That already exists:
https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca2249

[tarekgh]

CC @Youssef1313

[bartonjs]

The analyzer/fixer should identify both a.IndexOf(b) == 0 to a.StartsWith(b) and a.IndexOf(b) != 0 to !a.StartsWith(b). And generally apply for the overloads that take comparison options.

Category:Performance
Severity:Suggestion