G
G.Doten
Peter said:
I think you guys are right and I'm wrong on this one. At least I can't
get a class to do this (see my attempt below). I thought that
String.Intern would cause a single instance of my Empty field to be
created, but that doesn't seem to be the case, and I don't understand why.
The String class is integrated right into the CLR so that it knows the
exact layout of the class' fields (quoting Jeffrey Richter here), so I
bet if you look at the IL generated that there is special "string code"
that checks the one instance of the Empty field. But I'll admit I'm a
little puzzled on this one too.
I didn't say that the str2 was replaced or that str1's old string value
was replaced. Here's what Jon said:
and I said:
Indeed, str1 is replaced by a new instance of a string object, the one
returned by the call to str1.Remove. And the reference to the string
that str1 used to point to is decremented by 1 so when it goes to 0 the
GC can have at it.
I see. I guess for the same reason that the char[] constructor behaves
the way it does, because it is documented to. This is in the Remove
documentation: "returns a new String that is equivalent to this instance
less count number of characters." Notice there is no verbiage describing
special-casing around a string that happens to come back as empty; it
says it *always* returns a new string. Which makes sense to me given the
immutable nature of strings. I don't think I'd want the Remove method to
have that special case, but I can see it's usefulness (in terms of
performance) for the char[] constructor.
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Here's the code I tried:
using System;
namespace MyStringTest
{
class Program
{
static void Main()
{
MyString x = new MyString(new char[] { });
MyString y = new MyString(new char[] { });
Assert("case 1a", true, ReferenceEquals(x, y));
Assert("case 1b", true, ReferenceEquals((object)x, (object)y));
string s1 = new string(new char[] { });
string s2 = new string(new char[] { });
Assert("case2", true, ReferenceEquals(s1, s2));
string s = "Hello";
Assert("case2", true, ReferenceEquals("Hello", s));
string h1 = "Hel";
string h2 = "lo";
Assert("case3a", false, ReferenceEquals("Hello", h1+h2));
Assert("case3b", true, ReferenceEquals("Hello", string.Intern(h1+h2)));
}
static void Assert(string name, bool expected, bool actual)
{
Console.Write(name + " - ");
Console.WriteLine(expected == actual ? "success" : "failure");
}
}
public class MyString
{
public static readonly string Empty = string.Intern("");
string _s;
public MyString(char[] ch)
{
if (ch == null || ch.Length == 0)
_s = Empty;
else
_s = new string(ch);
}
public bool IsEmpty
{
get
{
return _s == Empty;
}
}
}
}
