C# ArrayList.Add Method Adds Ref to Items?

Question

0Suppose I have created an ArrayList and added two strings as such:

string item1 = string.Empty;
string item2 = string.Empty;

ArrayList someList = new ArrayList();
someList.Add(item1);
someList.Add(item2);

someFunc(someList);

Then I take someList and pass it into a method. That method performs some manipulation on item1 and item2 and returns, as such:

public void someFunc(ArrayList parameters)
{
    parameters[0] = "Monkeys";
    parameters[1] = "More Monkeys";
}

I would expect that when someFunc returns that item1 equals "Monkeys" and item2 equals "More Monkeys" as someList contains references to item1 and item2 in indices 0 and 1 respectively. However, what I am seeing is that item1 equals "Monkeys" and item2 equals "".

Why? And how would I perform this feat without passing in each item individually by ref?

"However, what I am seeing is that item1 equals "Monkeys" and item2 equals ""." Are you sure that's what you are seeing? Because you shouldn't see that at all. From the given snippets, both would be empty strings. — Anthony Pegram
– Anthony Pegram, Commented Mar 26, 2013 at 15:19
The code works for me. Have you stepped through it with a debugger? — Ryan Frame
– Ryan Frame, Commented Mar 26, 2013 at 15:19
@RyanFrame, what is your definition of "works" here? Doesn't throw? Assigns to the arraylist? His definition of "works" is having item1 and item2 reflect "Monkeys" and "More Monkeys". Is that what you see? If so, how? — Anthony Pegram
– Anthony Pegram, Commented Mar 26, 2013 at 15:21
item1 and item2 after your call to someFunc should both still be empty strings. Your someList[0] and someList[1] should be Monkeys and More Monkeys respectively. If you're seeing anything other than that, then there is something you are not telling us. — Matt Burland
– Matt Burland, Commented Mar 26, 2013 at 15:21
When I test your code, someList is changed (both values), item1 and item2 not. All as expected. — Hans Keﬆing
– Hans Keﬆing, Commented Mar 26, 2013 at 15:21

Community · Accepted Answer · 2017-05-23 11:49:56Z

Your code will simply not do what you expect. Your variables item1 and item2 will not be modified by reassigning to the list. You cannot add a reference to a variable to an ArrayList (or preferably a List<T> in the modern .NET world). You can simply add the value itself (which could be a reference to an object). When you overwrite the value that was originally stored in the list, it does not impact the previous variable.

If you need updates inside the list to reflect on outer variables, you may need a different abstraction. You make your variables properties of a class, for example, and then maintain a list of that class. And instead of overwriting the contents of your list, you could then mutate the class by changing the property. This technique is demonstrated in Niyoko Yuliawan's answer, so I will not redisplay it.

Another technique mentioned is to read the values back into your variables. If the list has the same number of values in the same order, this is doable. However, I should note that if you are adding multiple values to a list, passing them into a method, changing the values in the list, and then rereading those values into your variables, you probably did not need a list, you needed a class. If item1 and item2 are really firstName and lastName, for example, you don't want ArrayList list (or, again, preferably List<string> list), you really want Person person that has string FirstName and string LastName properties.

So if you had

public void SomeFunc(ArrayList list)
{
     list[0] = "Sam";
     list[1] = "Smith";
}

You probably should consider instead defining a class and then working with that class.

public class Person 
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

// elsewhere in your code...

public void SomeFunc(Person person)
{
     person.FirstName = "Sam";
     person.LastName = "Smith";
}

Niyoko · Accepted Answer · 2013-03-27 06:03:03Z

2

You can't do that. However, you can do it like this:

class Program
{
    class SomeClass
    {
        public string Content { get; set; }
    }

    static void Main(string[] args)
    {
        SomeClass a, b;
        a = new SomeClass { Content = "" };
        b = new SomeClass { Content = "" };
        ArrayList al = new ArrayList();
        al.Add(a);
        al.Add(b);

        SomeFunction(al);

        Console.WriteLine(a.Content);
        Console.WriteLine(b.Content);
        Console.ReadLine();
    }

    public static void SomeFunction(ArrayList param)
    {
        ((SomeClass)param[0]).Content = "aa";
        ((SomeClass)param[1]).Content = "bb";
    }
}

edited Mar 27, 2013 at 6:03

answered Mar 26, 2013 at 15:26

Niyoko

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Comments

Matt Burland · Accepted Answer · 2013-03-26 15:26:23Z

1

To get what you wanted, you'd have to read the values back from your ArrayList:

string item1 = string.Empty;
string item2 = string.Empty;

ArrayList someList = new ArrayList();
someList.Add(item1);
someList.Add(item2);

someFunc(someList);

item1 = (string)someList[0];
item2 = (string)someList[1];

BTW, there aren't too many cases where ArrayList is a better choice that List<T>, in this case, List<string>.

answered Mar 26, 2013 at 15:26

Matt Burland

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Comments

chris warner · Accepted Answer · 2013-03-26 15:37:35Z

You cant do this without reading them back from the Array, here is a LinqPad example to show you the output. As you can see the value of the array has changed but your original variables haven`t.

    void Main()
{
string item1 = "test1";
string item2 = "test2";

ArrayList someList = new ArrayList();
someList.Add(item1);
someList.Add(item2);

item1.Dump();
item2.Dump();

someFunc(someList);

item1.Dump();
item2.Dump();

someList.Dump();
}

public void someFunc(ArrayList parameters)
{
    parameters[0] = "Monkeys";
    parameters[1] = "More Monkeys";
}

using a "true" reference type wrapper works

public class StringWrapper
{
    public string value;
}

void Main()
{
StringWrapper item1 = new StringWrapper() {value = "test1" };
StringWrapper item2 = new StringWrapper() {value = "test2" };

List<StringWrapper> someList = new List<StringWrapper>();
someList.Add(item1);
someList.Add(item2);

item1.Dump();
item2.Dump();

someFunc(someList);

item1.Dump();
item2.Dump();

someList.Dump();
}

public void someFunc(List<StringWrapper> parameters)
{
    parameters[0].value = "Monkeys";
    parameters[1].value = "More Monkeys";
}

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