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How to Prove Immutabiltiy of String in C#?

In my last c# interview,
I was asked to prove immutability of C# string,I know what is meant by immutability of c# string,But is it possible to prove immutability of c# string through code ? can i have a sample code snippet please.
Thanks in advance

3 Answers
3

I can prove that a string is not immutable. All I need to do is to show some code which mutates a string, like so:

string

string

using System;
using System.Runtime.InteropServices;

namespace Demo

class Program

static void Main(string args)

const string test = "ABCDEF"; // Strings are immutable, right?
char chars = new StringToChar str = test.chr;
chars[0] = 'X';

// On an x32 release or debug build or on an x64 debug build,
// the following prints "XBCDEF".
// On an x64 release build, it prints "ABXDEF".
// In both cases, we have changed the contents of 'test' without using
// any 'unsafe' code...

Console.WriteLine(test);

// The following line is even more disturbing, since the constant
// string "ABCDEF" has been mutated too (because the interned 'constant' string was mutated).

Console.WriteLine("ABCDEF");



[StructLayout(LayoutKind.Explicit)]
public struct StringToChar

[FieldOffset(0)] public string str;
[FieldOffset(0)] public char chr;




Now whether this should be considered a bug in C# is a different matter. :)
(The answer is probably that FieldOffset should be considered to be unsafe - the code above is purportedly safe and therefore the string should not be mutatable.)

FieldOffset

unsafe

safe

string

Also, I think you could legitimately argue that string is immutable in spirit, even if there are silly edge cases which violate its immutability in supposedly safe code.

string

string

Console.WriteLine("ABCDEF");

String

str += "XXX"

str = string.Concat(str, "XXX")

+=

StringToChar

test

str

chr

FieldOffset(0)

Yes, It is possible to prove immutability of c# string using ObjectIDGenerator Class.

ObjectIDGenerator

Following answer is taken from dotmob article on String Vs Stringbuilder in C#

Actually ObjectIDGenerator will return an unique integer value for instances that we created in our programs.With the help of this class we can check whether new instance is created or not for various operations on string and stringbuilder .Consider following program

using System;
using System.Text;
using System.Runtime.Serialization;

class Program

static void Main(string args)

ObjectIDGenerator idGenerator = new ObjectIDGenerator();
bool blStatus = new bool();
//just ignore this blStatus Now.
String str = "My first string was ";
Console.WriteLine("str = 0", str);
Console.WriteLine("Instance Id : 0", idGenerator.GetId(str, out blStatus));
//here blStatus get True for new instace otherwise it will be false
Console.WriteLine("this instance is new : 0n", blStatus);
str += "Hello World";
Console.WriteLine("str = 0", str);
Console.WriteLine("Instance Id : 0", idGenerator.GetId(str, out blStatus));
Console.WriteLine("this instance is new : 0n", blStatus);
//Now str="My first string was Hello World"
StringBuilder sbr = new StringBuilder("My Favourate Programming Font is ");
Console.WriteLine("sbr = 0", sbr);
Console.WriteLine("Instance Id : 0", idGenerator.GetId(sbr, out blStatus));
Console.WriteLine("this instance is new : 0n", blStatus);
sbr.Append("Inconsolata");
Console.WriteLine("sbr = 0", sbr);
Console.WriteLine("Instance Id : 0", idGenerator.GetId(sbr, out blStatus));
Console.WriteLine("this instance is new : 0n", blStatus);
//Now sbr="My Favourate Programming Font is Inconsolata"
Console.ReadKey();




Output Will look like this

Instance id for string get changed from 1 to 2 when str concatenated with “Hello World”.while instance id of sbr remains same as 3 after append operation also. This tells all about mutability and immutability. blStatus variable indicate whether the instance is new or not.

You can find complete article on the topic from : http://dotnetmob.com/csharp-article/difference-string-stringbuilder-c/

It is possible to prove immutability of c# string

A simple example:

string str = "test";string str2 = str;Console.WriteLine(str2);

string str = "test";string str2 = str;Console.WriteLine(str2);

//output: test

str = "DDD";Console.WriteLine(str2);

str = "DDD";Console.WriteLine(str2);

//output: test

Try this code, if string is mutable, "str2" should be "DDD".

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