[Mono-bugs] [Bug 431453] New: internal GetEnumerator() methods should not be considered within a foreach()

[bugzilla_noreply at novell.com]

https://bugzilla.novell.com/show_bug.cgi?id=431453

User jpryor at novell.com added comment
https://bugzilla.novell.com/show_bug.cgi?id=431453#c2

           Summary: internal GetEnumerator() methods should not be
                    considered within a foreach()
           Product: Mono: Compilers
           Version: SVN
          Platform: x86-64
        OS/Version: openSUSE 11.0
            Status: NEW
          Severity: Normal
          Priority: P5 - None
         Component: C#
        AssignedTo: mono-bugs at lists.ximian.com
        ReportedBy: jpryor at novell.com
         QAContact: mono-bugs at lists.ximian.com
          Found By: ---


Consider the following source:

  using System;

  public class Enumerator
  {
    public bool MoveNext()
    {
      return false;
    }

    public int Current { get; set; }
  }


  public class Base
  {
    public Enumerator GetEnumerator()
    {
      return new Enumerator();
    }
  }

  public class C : Base
  {
    new internal Enumerator GetEnumerator()
    {
      return new Enumerator();
    }
  }

  class Test {
    public static void Main ()
    {
      foreach (var e in new C())
        Console.WriteLine (e);
    }
  }

Under CSC, compiling produces the following errors:

Microsoft (R) Visual C# 2008 Compiler version 3.5.21022.8
for Microsoft (R) .NET Framework version 3.5
Copyright (C) Microsoft Corporation. All rights reserved.

Documents\c.cs(33,9): warning CS0279: 'C' does not implement the 'collection'
pattern. 'C.GetEnumerator()' is either static or not public.
Documents\c.cs(24,29): (Location of symbol related to previous warning)
Documents\c.cs(33,9): error CS1579: foreach statement cannot operate on
variables of type 'C' because 'C' does not contain a public definition for
'GetEnumerator'

gmcs doesn't produce any error.

The reasoning is described in section 15.8.4 of the C# standard (4th edition),
"The foreach statement", page 237, major bullet #2, sub-bullet #2:

"Perform overload resolution using the resulting method group and an empty
argument list. If overload resolution results in no applicable methods, results
in an ambiguity, or results in a single best method but that method is either
static or not public, check for an enumerable interface as described below. It
is recommended that a warning be issued if overload resolution produces
anything except an unambiguous public instance method or no applicable
methods."

Because C.GetEnumerator() is internal, and because C doesn't implement
IEnumerable, then there is no .GetEnumerator() method which matches within the
scope of `foreach(var e in new C())`, so it should result in a failure.


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