Information about "in" function arguments missing

[ssa3512]

The Argument Lists section at https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/expressions#argument-lists references the ref and out parameter modifiers but is missing information on the newer in modifier which passes an item by reference but as read-only. The in modifier is described in the keywords section here: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/in-parameter-modifier

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Document Details

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• ID: e783650c-e063-e268-316e-564362cb1e37
• Version Independent ID: aa9f4996-cab4-0bed-6452-213342453785
• Content: Expressions - C# language specification
• Content Source: spec/expressions.md
• Product: dotnet-csharp
• Technology: csharp-spec
• GitHub Login: @dotnet-bot
• Microsoft Alias: wiwagn

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Associated WorkItem - 52212

[dotnet-bot]

This issue has been closed as part of the issue backlog grooming process outlined in dotnet/docs#22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.

[ssa3512]

@dotnet/docs in is still a valid C# argument modifier since C# 7.2, though this documentation has still not been updated to reflect this.

There is a page for in parameters here, but if this language specifications documentation is still being maintained, it needs to be updated.

[ssa3512]

@BillWagner tagging @dotnet/docs does not seem to work and I do not have an option to reopen as suggested by the dotnet-bot https://github.com/dotnet/docs/issues/17084#issuecomment-766878275

[BillWagner]

I believe this will be addressed in #219

[KalleOlaviNiemitalo]

The C# standard should specify that the following does not throw ArrayTypeMismatchException at run time:

static class Program {
    static void Main() {
        object[] array = new string[1];
        Sub(in array[0]); // Does not throw ArrayTypeMismatchException; the run-time type check is suppressed.
    }
    
    static void Sub(in object elt)
    {
        // Assigning elt = new object() here would violate type safety,
        // but the in modifier disallows this assignment.
    }
}

SharpLab shows that the IL code uses a readonly. prefix on the ldelema opcode to suppress the runtime check:

.class private auto ansi '<Module>'
{
} // end of class <Module>

.class private auto ansi abstract sealed beforefieldinit Program
    extends [System.Private.CoreLib]System.Object
{
    // Methods
    .method private hidebysig static 
        void Main () cil managed 
    {
        // Method begins at RVA 0x2050
        // Code size 22 (0x16)
        .maxstack 2
        .locals init (
            [0] object[]
        )

        IL_0000: ldc.i4.1
        IL_0001: newarr [System.Private.CoreLib]System.String
        IL_0006: stloc.0
        IL_0007: ldloc.0
        IL_0008: ldc.i4.0
        IL_0009: readonly.
        IL_000b: ldelema [System.Private.CoreLib]System.Object
        IL_0010: call void Program::Sub(object&)
        IL_0015: ret
    } // end of method Program::Main

    .method public hidebysig static 
        void Sub (
            [in] object& elt
        ) cil managed 
    {
        .param [1]
            .custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.IsReadOnlyAttribute::.ctor() = (
                01 00 00 00
            )
        // Method begins at RVA 0x2072
        // Code size 1 (0x1)
        .maxstack 8

        IL_0000: ret
    } // end of method Program::Sub

} // end of class Program

Alas, this code is not verifiable according to ECMA-335 6th Edition. §II.1.8.1.2.2 (Controlled-mutability managed pointers) says the result of readonly. ldelema "cannot be passed as a byref argument to a method." The verifier ignores the [in] ParamAttr in [in] object& elt because §II.15.4 (Defining methods) says "The CLI itself does not enforce the semantics of these bits".