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More cases to optimize linq #1157

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@jamesHargreaves12 jamesHargreaves12 commented Aug 14, 2023

Adds support for the following transformations:

OrderBy(...).FirstOrDefault() => MinBy(...)
OrderByDescending(...).FirstOrDefault() => MaxBy(...)

Do not restrict Applying Demorgans laws to the cases when the inside of the All / Any invocation is of the form !(...) - We can invert any boolean expression using SyntaxLogicalInverter.

@josefpihrt
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@jamesHargreaves12 MinBy/MaxBy throws exception in case of no elements so it should be First not FirstOrDefault.

@@ -199,6 +213,48 @@ public static void AnalyzeWhere(SyntaxNodeAnalysisContext context, in SimpleMemb
Report(context, invocation, span, checkDirectives: true, properties: properties);
}

private static bool IsNet6OrGreater(Compilation compilation)
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This is not maintainable as .NET version will grow.

I suggest to check if the MinBy method is contained in Enumerable class, for example:

enumerableSymbol.FindMember<IMethodSymbol>("MinBy") is not null

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I agree thats much better :)

@@ -184,6 +184,7 @@ private static void AnalyzeInvocationExpression(SyntaxNodeAnalysisContext contex
{
OptimizeLinqMethodCallAnalysis.AnalyzeWhere(context, invocationInfo);
OptimizeLinqMethodCallAnalysis.AnalyzeFirstOrDefault(context, invocationInfo);
OptimizeLinqMethodCallAnalysis.AnalyzerOrderByAndFirstOrDefault(context, invocationInfo);
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Move to First case.

@jamesHargreaves12
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I don't think MaxBy does throw in the case of an empty List:
https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.minby?view=net-6.0
https://dotnetfiddle.net/4cdut9

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Ok, it does not throw always when the sequence is empty but sometimes it does:

https://source.dot.net/#System.Linq/System/Linq/Max.cs,442

/// <para>If <typeparamref name="TKey" /> is a reference type and the source sequence is empty or contains only values that are <see langword="null" />, this method returns <see langword="null" />.</para>

https://dotnetfiddle.net/BxHq1P

@jamesHargreaves12
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I have messed about with this some more and the Csharp docs are wrong it should be TSource instead of TKey which is checked for reference type.

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You are right that the doc is wrong, but the bottom line is that MaxBy throws sometimes which must be taken into account when applying these LINQ optimizations.

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