What about Nullable struct?

[gpetrou]

Should there be some information here on whether to use or not the in-modifier with Nullable struct?
For example should we have
public void Method(int? arg) {}
or
public void Method(in int? arg) {}
?
ErrorProne.NET for example flags this as a special case: https://github.com/SergeyTeplyakov/ErrorProne.NET/blob/5cc962abfaf0005890dd0a9fba46a9e304a8a450/src/ErrorProne.NET.StructAnalyzers/TypeExtensions.cs#L28

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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
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

That's a great question @gpetrou

I'd have to benchmark that case.

Ping @jaredpar if the C# team has any general guidance on nullable structs as in parameters.

[jaredpar]

They should not be marked as readonly because a Nullable<T> is a struct which is non-readonly. There was an attempt to make Nullable<T> into a readonly struct but that was a breaking change. The reasoning is detailed here:

dotnet/corefx#24997 (comment)

In short though the problem is the backing T was never marked readonly and making it readonly now can cause observable side effects. Consider for example a ToString implementation that memoized the result. Before that was cached in the Nullable<T> and now it's not. Hence it remains a standard struct and should follow the same rules.

[gpetrou]

Thanks for the explanation. I will open an issue in Error.Prone repository to fix this.