Using C# 7.2 in modifier for parameters with primitive types

Question

C# 7.2 introduced the in modifier for passing arguments by reference with the guarantee that the recipient will not modify the parameter.

This article says:

You should never use a non-readonly struct as the in parameters because it may negatively affect performance and could lead to an obscure behavior if the struct is mutable

What does this mean for built-in primitives such as int, double?

I would like to use in to express intent in code, but not at the cost of performance losses to defensive copies.

Questions

• Is it safe to pass primitive types via in arguments and not have defensive copies made?
• Are other commonly used framework structs such as DateTime, TimeSpan, Guid, ... considered readonly by the JIT?
  • If this varies by platform, how can we find out which types are safe in a given situation?

Answer 1

A quick test shows that, currently, yes, a defensive copy is created for built-in primitive types and structs.

Compiling the following code with VS 2017 (.NET 4.5.2, C# 7.2, release build):

using System;

class MyClass
{
    public readonly struct Immutable { public readonly int I; public void SomeMethod() { } }
    public struct Mutable { public int I; public void SomeMethod() { } }

    public void Test(Immutable immutable, Mutable mutable, int i, DateTime dateTime)
    {
        InImmutable(immutable);
        InMutable(mutable);
        InInt32(i);
        InDateTime(dateTime);
    }

    void InImmutable(in Immutable x) { x.SomeMethod(); }
    void InMutable(in Mutable x) { x.SomeMethod(); }
    void InInt32(in int x) { x.ToString(); }
    void InDateTime(in DateTime x) { x.ToString(); }

    public static void Main(string[] args) { }
}

yields the following result when decompiled with ILSpy:

...
private void InImmutable([System.Runtime.CompilerServices.IsReadOnly] [In] ref MyClass.Immutable x)
{
    x.SomeMethod();
}

private void InMutable([System.Runtime.CompilerServices.IsReadOnly] [In] ref MyClass.Mutable x)
{
    MyClass.Mutable mutable = x;
    mutable.SomeMethod();
}

private void InInt32([System.Runtime.CompilerServices.IsReadOnly] [In] ref int x)
{
    int num = x;
    num.ToString();
}

private void InDateTime([System.Runtime.CompilerServices.IsReadOnly] [In] ref DateTime x)
{
    DateTime dateTime = x;
    dateTime.ToString();
}
...

(or, if you prefer IL:)

IL_0000: ldarg.1
IL_0001: ldobj [mscorlib]System.DateTime
IL_0006: stloc.0
IL_0007: ldloca.s 0
IL_0009: call instance string [mscorlib]System.DateTime::ToString()
IL_000e: pop
IL_000f: ret