C# strange behaviour in foreach loop

Question

After a day of troubleshooting I've managed to condense the problem to this tiny piece of code. Could someone explain to me why this doesn't work? I expect [markets] to be 0 2 4 6, [city] [county] and [streets] to be 0 1 2 3 when the messagebox is shown.

        private void pieceoftestcode()
        {
            string[] county = new string[4];
            string[] city = new string[4];
            string[] markets = new string[4];
            string[] streets = new string[4];
            string[] items = new string[4] { "apple", "banana", "pineapple", "juice" };
            string[] value = new string[4];
            foreach (string item in items)
            {
                for (int i = 0; i <= 3; i++)
                {
                    if (item == "apple")
                        value[i] = (2 * i).ToString();
                    else
                        value[i] = i.ToString();
                }

                if (item == "apple")
                    markets = value;
                else if (item == "banana")
                    streets = value;
                else if (item == "pineapple")
                    county = value;
                else
                    city = value;
            }
            MessageBox.Show("test");
        }

I'm looping through items in a foreach loop. If the item is "apple", then I expect [value] to be 0 2 4 6. Initially [markets] is assigned 0 2 4 6. However, if I execute the code step by step, it appears that the second time that the foreachloop is executed, [markets] is being overwritten. Why is that? What am I doing wrong here? [markets] should not be assigned a value a second time once banana has hit right?

Answer 1

You gradually end up with all of your various variables referencing the same array (value), with whatever values are written into that array by the last iteration being set.

There's a very similar way of writing this code that avoids the issue:

    private void pieceoftestcode()
    {
        string[] county = new string[4];
        string[] city = new string[4];
        string[] markets = new string[4];
        string[] streets = new string[4];
        string[] items = new string[4] { "apple", "banana", "pineapple", "juice" };
        string[] value;
        foreach (string item in items)
        {
            if (item == "apple")
                value = markets;
            else if (item == "banana")
                value = streets;
            else if (item == "pineapple")
                value = county;
            else
                value = city;
            for (int i = 0; i <= 3; i++)
            {
                if (item == "apple")
                    value[i] = (2 * i).ToString();
                else
                    value[i] = i.ToString();
            }


        }
        MessageBox.Show("test");
    }

Now, each time through the loop value get assigned a reference to a different array¹ and so the for loop doesn't overwrite its previous efforts.

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¹Assuming items doesn't contain any duplicate items nor more than one non-apple, -banana or -pineapple item.