String class behavior

Question

   public void main()
   {    
    string test = "testing";
    ChangeVal(test);
    Console.WriteLine(test);
   }
private void ChangeVal(string test)
{
    test = "in child";
}

If String is a class. and i pass string as a parameter to a function. change the value of that string in function. But in main function it shows the previous values. It will print testing value.

when i created Foo class which has 2 member variable integer and string. when i passed the object of the class as parameter and change value of the member variable in function. It will give updated value in the main function

public class Foo
{
    public string test = "testing";
    public int i = 5;

}

public void main()
{
        Foo obj=new Foo();
        Console.WriteLine(obj.test);
        ChangeVal(obj);
        Console.WriteLine(obj.test);
}

private void ChangeVal(Foo obj)
{
    obj.test = "in child";
    obj.i = 5;
}

If string is the class. It will update the value of the variable. May string is the sequence of Unicode character that's why it doesn't update the value in 1st case. Can any body will explain this in detail.

In your second example you are changing the properties but in the first example you are flat out assigning the argument, can't you see the difference? — Esailija
– Esailija, Commented Nov 20, 2012 at 12:16
Ya i can see the difference, but my question if string is the class and i had change the value of class variable then it should be reflected — Rahul Rajput
– Rahul Rajput, Commented Nov 20, 2012 at 12:24
@RahulRajput, in your 2nd example in ChangeVal method do: obj = null, see if you get the change in the main method, — Habib
– Habib, Commented Nov 20, 2012 at 12:29

Oded · Accepted Answer · 2012-11-20 12:17:23Z

3

change the value of that string in function

Strings are immutable. You can't change the value of a string. You can assign another string to the same reference, but you would need to pass the reference in by using ref.

public void main()
{    
  string test = "testing";
  ChangeVal(ref test);
  Console.WriteLine(test);
}

private void ChangeVal(ref string test)
{
    test = "in child";
}

You Foo class, however, is mutable, so you can assign different values to its members.

answered Nov 20, 2012 at 12:17

Oded

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4 Comments

Rahul Rajput Over a year ago

as i had passed test it is nothing but the reference and it is pointing to "testing" in the memory. when i re-initialize the string it will create new reference and test will now point to the "in child" in memory am i right? without ref so how come when i go back to main function it will again print the "testing" ?

Oded Over a year ago

@RahulRajput - When you use ref, yes.

Rahul Rajput Over a year ago

can you tell me internal memory representation? that will be helpful

Oded Over a year ago

@RahulRajput - No, as this is an implementation detail and can change between runtime releases.

Carlos Landeras · Accepted Answer · 2012-11-20 12:18:26Z

2

Try to pass the parameter by reference to get the var updated in main thread:

private void SeString(ref string chain)
        {
            chain="new string";

        }

Then call:

string variable="hello";
SeString(ref  variable);

string output is "new string"

answered Nov 20, 2012 at 12:18

Carlos Landeras

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