v7.x Q.s re the "ref local and returns" spec

[RexJaeschke]

In Draft PR #213, I've spec'd "ref locals and returns." This issue asks some questions that might impact that spec.

Question 1:

Given the following method declarations:

public static int M1() { … }
public static ref int M2() { … }
public static ref readonly int M3() { … }

and the following ref-assignment:

int i;
i = M1() - M2() + M3();

the values returned from each method are used to compute the result that is stored in i. However, M2 and M3 actually return a reference to a writeable and read-only int, respectively. Is there an implicit conversion going on here? That is, when one has a reference, but a value is expected, the value is taken. I ask, as my spec does not mention any such conversion.

Question 2:

Given

foreach_statement
    : 'foreach' '(' local_variable_type identifier 'in' expression ')' embedded_statement
    ;

it seems like local_variable_type could/should allow a ref or ref readonly prefix, as follows:

foreach_statement
    : 'foreach' '(' (ref readonly?)? local_variable_type identifier 'in' expression ')' embedded_statement
    ;

While my compiler allows these keywords there, I was not able to get an example to compile. Can someone show me correct code examples using ref and ref readonly, so I can write text for this case?

Question 3:

In 13.11, "The try statement," we have

...
catch_clause
    :  'catch' exception_specifier?  block
    ;

exception_specifier
    : '(' type identifier? ')'
    ;

The compiler does not allow ref or ref readonly prior to type. Should it? That is, can we catch-by-ref?

[KalleOlaviNiemitalo]

Re question 3, I don't think catch-by-ref should be allowed. If it were allowed, I imagine it could be used for:

• Not making a copy of the caught value.
  • The type in exception_specifier must be derived from System.Exception and thus a reference type, so it's cheap to copy anyway.
• Reassigning the reference, to make a subsequent throw; rethrow an exception instance different from the one that was caught.
  • It is not clear how that should affect Exception.StackTrace.
  • Would need to change ECMA-335 6th ed. §II.19 (Exception handling) and §II.25.4.6 (Exception handling clauses) to describe how the catch block is marked to give it a managed pointer rather than a reference.
  • One can already use throw expression for a similar effect.

.NET SDK 5.0.203 apparently allows returning by reference in property_declaration and indexer_declaration but not operator_declaration. I haven't found a rationale for that.

[KalleOlaviNiemitalo]

Re question 2:

using System;

class ForeachRefDemo
{
    static void Main()
    {
        int[] array = new int[5];

        foreach (ref int i in array.AsSpan())
        {
            i = 42;
        }

        Console.WriteLine(array[2]);
    }
}

Or without using Span:

using System;

class ForeachRefDemo
{
    static void Main()
    {
        int[] array = new int[5];

        foreach (ref int i in new IntArrayRefEnumerable(array))
        {
            i = 42;
        }

        Console.WriteLine(array[2]);
    }
}

struct IntArrayRefEnumerable
{
    int[] array;

    public IntArrayRefEnumerable(int[] array)
    {
        this.array = array;
    }

    public IntArrayRefEnumerator GetEnumerator() => new IntArrayRefEnumerator(this.array);
}

struct IntArrayRefEnumerator
{
    int[] array;
    int index;

    public IntArrayRefEnumerator(int[] array)
    {
        this.array = array;
        this.index = -1;
    }

    public bool MoveNext()
    {
        if (this.index < this.array.Length - 1)
        {
            this.index++;
            return true;
        }
        else
        {
            return false;
        }
    }

    public ref int Current => ref this.array[this.index];
}

[RexJaeschke]

Re Question 1: This is answered in 12.2.2 Values of expressions:

Most of the constructs that involve an expression ultimately require the expression to denote a value. In such cases, if the actual expression denotes a namespace, a type, a method group, or nothing, a compile-time error occurs. However, if the expression denotes a property access, an indexer access, or a variable, the value of the property, indexer, or variable is implicitly substituted:

• The value of a variable is simply the value currently stored in the storage location identified by the variable. A variable shall be considered definitely assigned (§10.4) before its value can be obtained, or otherwise a compile-time error occurs.
  ...

Re Question 2: Thanks, @KalleOlaviNiemitalo for the working example. I have added support for ref readonly to foreach.

Re Question 3: @KalleOlaviNiemitalo answered this. No change is needed.