Don't pass a nullable value type as an in argument. The Nullable type isn't declared as a read-only struct. That means the compiler must generate defensive copies for any nullable value type argument passed to a method using the in modifier on the parameter declaration.

I think this guidance is a bit too strong. Roslyn (v3.2 and later) does not make defensive copies when accessing some members of Nullable<T> even when targeting .NET Framework (where none of its members are readonly).

Document Details

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• ID: ac8804c7-6acd-ab2a-4e14-0b99265969a5
• Version Independent ID: 183efcbd-a781-d303-f9cd-1c22da1676de
• Content: Write safe and efficient C# code
• Content Source: docs/csharp/write-safe-efficient-code.md
• Product: dotnet-csharp
• Technology: csharp-advanced-concepts
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

Associated WorkItem - 54744