[Mono-bugs] [Bug 395904] New: StringBuilder.ToString(int, int) assumes that String. Substring always creates a new object

[bugzilla_noreply at novell.com]

https://bugzilla.novell.com/show_bug.cgi?id=395904


           Summary: StringBuilder.ToString(int, int) assumes that
                    String.Substring always creates a new object
           Product: Mono: Class Libraries
           Version: SVN
          Platform: Other
        OS/Version: Other
            Status: NEW
          Severity: Normal
          Priority: P5 - None
         Component: CORLIB
        AssignedTo: mono-bugs at lists.ximian.com
        ReportedBy: juraj at hotfeet.ch
         QAContact: mono-bugs at lists.ximian.com
          Found By: ---


Created an attachment (id=219223)
 --> (https://bugzilla.novell.com/attachment.cgi?id=219223)
minimal test case

This bug in StringBuilder is the reason why this code in String.Substring is
disabled:

// FIXME: this causes regressions in System.Xml
/*
if (startIndex == 0 && length == this.length)
   return this;
*/

To expose the bug:

- enable the code above, "make install PROFILE=net_2_0" in mcs/class/corlib
- compile and run the attached test case

Expected output: none
Actual output:   "Immutable string was changed!"


The source of the problem is that StringBuilder.ToString(int, int) implicitly
assumes that String.Substring will _always create a new string object_. With
the code above enabled, this is no longer the case.

In my opinion, we should add an internal method (e.g. "CopySubstring") to
String which is guaranteed to always create a new string object and call that
one from StringBuilder.ToString(int, int).


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