递归中未修改字符串值

Question

I am trying to modify a string inside a recursion. Here is the code:-

using System;


public class Program
{
    public static void Main()
    {
        string s = "abcdc";
        string added = "";
        checkIfPalindrome(s , added);
        Console.WriteLine( added);
    }

    public static void checkIfPalindrome(string s ,  string added)
    {
        //Console.WriteLine(added);
        if(s.Length < 2)
        {
            return;
        }
        if(s[0] == s[s.Length -1])
        {           
            checkIfPalindrome(s.Substring(1,s.Length-2) , added);       
        }
        else
        {           
            added = s[0] + added;           
            checkIfPalindrome(s.Substring(1,s.Length-1), added) ;
        }

    }
}

Here is the fiddle for the same:-

 <iframe width="100%" height="475" src="https://dotnetfiddle.net/Widget/u4119h" frameborder="0"></iframe>

I am expecting added to be modified inside recursion and appear as a result outside function call and inside main method, since string is a reference type.But the value of added literal is an empty string. Although when recursion hits the base case, it has ba as value.

Also correct value is reflected when ref keyword is used for passing added

Is my understanding wrong?

Answer 1

At the very top, you're assigning a value to the added local in your Main function. But you're never changing that value. You're only ever changing what the argument of checkIfPalindrome points to. This has no effect on the local added in Main , unless you pass it by reference - using ref .

Reference types have reference semantics, yes. But that doesn't mean that a reference to a reference type has the same property. If you modified the value of added , you would get the behaviour you expect - but you only ever replace the argument's reference - pointing it somewhere else. Of course, strings in .NET are immutable, so you can't actually change the value of added - your only option is to pass the string by reference (or better, make it a return value, which makes for much clearer recursion anyway).

Answer 2

string is a reference type but immutable which means it cannot be changed after it has been created. Every change to a string will create a new string.