Using static variable in C#

Question

I have problem when using static variable in my project (force using static variable)

public static List<int> a = new List<int>();
public static List<List<int>> list = new List<List<int>>();
public Form1()
{
    for (int i = 0; i < 5;i++ )
        a.Add(i);
    list.Add(a);
    Console.WriteLine(list[0].Count); // **count = 5**
    a.RemoveAt(0);
    list.Add(a);
    Console.WriteLine(list[0].Count); // **count = 4** 
    Console.WriteLine(list[1].Count); // count = 4
}

When I use a.RemoveAt(0) , it makes list[0] change. Why does it do this and how can I fix it?

because list[0] contains a so if you remove sth from a list[0].Count is different. It's not because of static, it's because list[0] contains reference to object a — Kamil Budziewski
– Kamil Budziewski, Commented Jul 12, 2013 at 12:38
Well yeah, you're referring to the same object. If you want to store a copy of a in list[0], then make a copy: list.Add(a.ToList()); (make sure to add a using System.Linq; directive) — Chris Sinclair
– Chris Sinclair, Commented Jul 12, 2013 at 12:39
Like @ChrisSinclair said, you need to make a copy. But please explain what you're doing -- why do you have a list of lists if you don't want them to actually refer to the source lists? Are you sure you don't just want a straight list? You can add all the elements of one list to another with AddRange. — nmclean
– nmclean, Commented Jul 12, 2013 at 12:47

Chris Sinclair · Accepted Answer · 2013-07-12 12:54:21Z

5

Well yeah, you're referring to the same object because List<T> is a reference type. See: http://msdn.microsoft.com/en-us/library/s6938f28.aspx

For example:

List<int> a = new List<int>();
List<int> b = a;
Console.WriteLine(Object.ReferenceEquals(a, b)); //true

a.Add(1);
Console.WriteLine(a[0]); //1
Console.WriteLine(b[0]); //1

a[0] = 9000;
Console.WriteLine(a[0]); //9000
Console.WriteLine(b[0]); //9000

Storing a list in a list will yield the same result: you are pointing to the same original list.

If you want to store a copy of a in list[0], then make a copy:

list.Add(new List<int>(a));

Or use linq to make it more succinct:

list.Add(a.ToList());

(make sure to add a using System.Linq; directive to the top of your code file)

edited Jul 12, 2013 at 12:54

answered Jul 12, 2013 at 12:42

Chris Sinclair

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Comments

Sandy · Accepted Answer · 2013-07-12 12:59:46Z

You must understand it from basics. Lists objects work through reference. When you added object a to list, that means you added a reference of a to list. Now what ever you change in a that will be reflected in list[0] also as it is referring to same reference.

To achieve this, you can do some like this.

        var masterList = new List<List<int>>();

        var l1 = new List<int>{1, 2, 3, 4, 5}; // Reference created for l1
        var l2 = new List<int>(); // Reference created for l2

        masterList.Add(l1); // l1 reference added to masterList
        masterList.Add(l2); // l2 reference added to masterList

        l2.AddRange(l1); // This will copy values from l1 reference to l2 reference and will not touch the references

        l2.RemoveAt(0); // First value removed from reference l2 (And therefore it should not affect reference l1)

        MessageBox.Show(masterList[0].Count.ToString() + " and " + masterList[1].Count.ToString());

It must help you to understand whats happening here. You must also remember that it has NOTHING to do with static variables as your question heading indicates.

Hope it helps.

Serberuss · Accepted Answer · 2013-07-12 12:41:03Z

1

list is keeping a reference to a so if you change the object a then the count in list will also change because it refers to the same thing.

What you need to do is make a copy of a and pass that into list, allowing list to keep a separate reference to a and allowing you to keep the count the same.

Example:

list.Add(a.ToList()); // See Chris Sinclair's example, full credit to him

answered Jul 12, 2013 at 12:41

Serberuss

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2 Comments

Chris Sinclair Over a year ago

This doesn't actually make a copy of a; it again is pointing to the same reference.

Serberuss Over a year ago

You're right I apologise, I've updated referring to your answer

terrybozzio · Accepted Answer · 2013-07-12 12:46:51Z

0

you need to make copy of list(because list is keep+ing a reference to a):

        public static List<int> a = new List<int>();
        public static List<List<int>> list = new List<List<int>>();

        for (int i = 0; i < 5; i++)
            a.Add(i);
        list.Add(a.Select(i => i).ToList());//passed in a copy of a.
        Console.WriteLine(list[0].Count); // **count = 5**
        a.RemoveAt(0);
        list.Add(a);
        Console.WriteLine(list[0].Count); // **count = 5** 
        Console.WriteLine(list[1].Count); // count = 4

answered Jul 12, 2013 at 12:46

terrybozzio

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