list of array of strings in c#, how to properly use the add method

Question

I don't get why this code's output is :

34
34

List<String[]> list = new List<String[]>();
String[] s = new String[2];

s[0] = "1";
s[1] = "2";

list.Add(s);

s[0] = "3";
s[1] = "4";

list.Add(s);

foreach (String[] match in list)
    Console.WriteLine(match[0] + match[1]);

I think my code is fine to get the output of

12
34

Can someone help?

Because you are printing the same array twice. Arrays are reference types. — afaolek, Jul 17 '14 at 16:40
... or [this](http://stackoverflow.com/questions/24775086/none-static-nested-class-inside-static-class-c-sharp) or [this](http://stackoverflow.com/questions/16696448/how-to-make-a-copy-of-an-object-in-c-sharp) or [this](http://stackoverflow.com/questions/78536/deep-cloning-objects) or [this](http://stackoverflow.com/questions/5843958/how-to-make-a-copy-of-a-reference-type) or [this](http://stackoverflow.com/questions/20698361/cloning-a-reference-type) ... — O. R. Mapper, Jul 17 '14 at 16:42
... or [this](http://stackoverflow.com/questions/15275251/how-save-array-in-list-by-value-not-reference-in-c-sharp) or [this](http://stackoverflow.com/questions/24157588/objects-in-listt-reference-same-value) or [this](http://stackoverflow.com/questions/22899129/how-to-add-an-array-to-a-list-by-value-not-by-reference) or [this](http://stackoverflow.com/questions/4736947/arraylist-add-adds-just-references) ... — O. R. Mapper, Jul 17 '14 at 16:46
When the site asks you to check whether your problem has been discussed before, **please do so**. SO is most useful if all the knowledge on every single problem is gathered in *one* question. — O. R. Mapper, Jul 17 '14 at 16:47
Would you expect the output to be "1 2" if you wrote `string s = "1"; s = "2"; Console.WriteLine(s); Console.WriteLine(s);`? — Patrick, Jul 17 '14 at 16:54

Patrick Hofman · Answer 1 · 2014-07-17T18:25:50.877

3

You are adding the same string array to the list twice. You are only changing the internal values of the array.

You have to initialize a new array to put the new values in:

String[] s = new String[2];

s[0] = "1";
s[1] = "2";

list.Add(s);

s = new String[2]; // initialize new array
s[0] = "3";
s[1] = "4";

list.Add(s);

edited Jul 17 '14 at 18:25

answered Jul 17 '14 at 16:40

Patrick Hofman

153,850
22
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Or use a different variable, like `s1`, and `s2`. Personal preference though. – gunr2171 Jul 17 '14 at 16:41
That may indeed be clearer, but there is no real need to. – Patrick Hofman Jul 17 '14 at 16:41
or `list.Add(new[]{ "1", "2"});` (since there is no need to use variables in the first place) – default Jul 17 '14 at 16:46
@Default: I think for a novice user this might me too much. – Patrick Hofman Jul 17 '14 at 16:54
I think some people think it's a good idea to [downvote answers to closed questions, more so for duplicates](http://meta.stackoverflow.com/q/265165/372643), – Bruno Jul 17 '14 at 17:26
@remundo: need more help? If not, please don't forget to accept the answer you liked most. – Patrick Hofman Jul 17 '14 at 22:28

Vlad from Moscow · Answer 2 · 2014-07-17T17:08:03.637

An array is a reference type. The list contains the same references as array s. So the both elements of the list are the same references as s. As the last value of s is { "3", "4" } then you get the result as is.

Try the following code

using System;
using System.Collections.Generic;

namespace ListOfArrays
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string[]> list = new List<string[]>();

            list.Add( new string[] { "1", "2" } );
            list.Add( new string[] { "3", "4" } );

            foreach (string[] match in list)
            {
                Console.WriteLine(match[0] + match[1]);
            }
        }
    }
}

The output is

12
34

Take into account that it is better to use type name string instead of String The last is dependent of using directive.

list of array of strings in c#, how to properly use the add method

2 Answers2