C# int array automatically copying other array

Question

I wrote this code that fills an array and a second array copies the first. I change some numbers in the second array, but when I output both arrays, the first one has also changed. This is my code:

        int[] array1 = new int[5];
        int[] array2 = new int[5];
        int temp;

        for (int i = 0; i < array1.Length; i++)
        {
            array1[i] = i;
        }

        array2 = array1;
        temp = array2[2];
        array2[2] = array2[4];
        array2[4] = temp;

        for (int i = 0; i < array1.Length; i++)
        {
            richTextBox1.Text += array1[i].ToString() + " ";
        }
        for (int i = 0; i < array2.Length; i++)
        {
            richTextBox1.Text += array2[i].ToString() + " ";
        }

Can someone explain to me why the output is 0 1 4 3 2 0 1 4 3 2 instead of 0 1 2 3 4 0 1 4 3 2?

Michael Liu · Accepted Answer · 2012-06-03 14:59:46Z

4

Because arrays are reference types, assigning array2 = array1 causes the array2 variable to reference the same array as array1.

Before assignment:

array1  --->  [0, 1, 2, 3, 4]

array2  --->  [0, 0, 0, 0, 0]

After assignment:

array1  --->  [0, 1, 2, 3, 4]
         /
array2  -     [0, 0, 0, 0, 0] <-- no longer reachable and will be garbage collected

To actually create a copy of an array, call Clone.

array2 = (int[])array1.Clone();

After assignment with Clone:

array1  --->  [0, 1, 2, 3, 4]

array2  -     [0, 0, 0, 0, 0] <-- no longer reachable and will be garbage collected
         \
          ->  [0, 1, 2, 3, 4]

Notice that the [0, 0, 0, 0, 0] array is never actually used, so you don't even need to allocate it.

//int[] array2 = new int[5];  <-- delete this line
int[] array2 = (int[])array1.Clone();

edited Jun 3, 2012 at 14:59

answered Jun 3, 2012 at 14:46

Michael Liu

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Comments

SLaks · Accepted Answer · 2012-06-03 14:46:34Z

3

array2 = array1;

You now have two variables that refer to the same array.

answered Jun 3, 2012 at 14:46

SLaks

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BrokenGlass · Accepted Answer · 2012-06-03 14:47:10Z

3

array2 = array1;

You are not copying the array - Array is a reference type and you are assigning another variable to point to the same array - there is only one array instance, so if you change its contents through one variable these changes are perceived through the other.

answered Jun 3, 2012 at 14:47

BrokenGlass

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Mouliyan · Accepted Answer · 2012-06-03 15:02:34Z

2

Because Arrays are reference type as others said.

You can use this instead:

Array.Copy(array1, array2, array2.Length);

answered Jun 3, 2012 at 15:02

Mouliyan

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bluish · Accepted Answer · 2012-06-04 07:46:39Z

2

array2 = array1;   // here you are losing reference to array2

edited Jun 4, 2012 at 7:46

bluish

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answered Jun 3, 2012 at 14:46

0lukasz0

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