Deep Copy with Array

Question

Why doesn't ICloneable's Clone method return a deep copy?

Here is some sample code:

class A : ICloneable 
{
    public int x = 2;
    public A(int x)
    {
       this.x = x; 
    }
       
    public A copy()
    {
        A a = new A(this.x);
        return a; 
    }

     public object Clone()
     {
         A a = new A(this.x);
         return a;
     }
}

In the Main method :

A[] Array1 = new A[4] ;
Array1[0] = new A(0);
Array1[1] = new A(1);
Array1[2] = new A(2);
Array1[3] = new A(3);
A[] Array2 = new A[10];
Array. Copy(Array1, Array2, 4); 

Array2[2].x = 11; 
for (int i = 0; i < 4; i++)
    Console.WriteLine($"Index {i} value: {Array1[i].x}");

Remembering that I've only changed element index 2 in Array2, here is the output from the code above, listing the values in Array1:

Index 0 value: 0
Index 1 value: 1
Index 2 value: 11
Index 3 value: 3

Index 2 in Array1 has 11 even though I changed it in Array2 and class A implements ICloneable!

What then is the benefit of Array implementing ICloneable?

Elveraelves

Answer 1

From http://msdn.microsoft.com/en-us/library/k4yx47a1.aspx:

"If sourceArray and destinationArray are both reference-type arrays or are both arrays of type Object, a shallow copy is performed. A shallow copy of an Array is a new Array containing references to the same elements as the original Array. The elements themselves or anything referenced by the elements are not copied"

There may be a better method than this, but one technique you could use is:

  A[] array2 = array1.Select (a =>(A)a.Clone()).ToArray();

Strander

But important to note in the general case that implementing ICloneable doesn't guarantee clones will be deep copies: "The ICloneable interface simply requires that your implementation of the Clone() method return a copy of the current object instance. It does not specify whether the cloning operation performs a deep copy, a shallow copy, or something in between." – Bydgoszcz

Answer 2

Array.Copy copies the values of the array, in this case, references. There is nothing in the documentation of Array.Copy() that indicates a check for classes that implement IClonable and call Clone() instead. You will need to loop through the array and call Clone() yourself.

BTW, yes, IClonable kind of sucks.

Unexceptional

Answer 3

Array.Copy() does not use ICloneable. It simply copies values stored in each cell (which in this case are references to your A objects)

Elinoreeliot

Answer 4

Ani's answer uses LINQ thus does not work for C# 2.0, however a same method can be done using the Array class's ConvertAll method:

A[] Array2 = Array.ConvertAll(Array1,a => (A)a.Clone());

Salinometer

Answer 5

This is an example of a deep copy using generic System.Array (and copying it to a new array that is twice as large):

    public Array ResizeArray( Array array1) {
        var size = array1.Length;
        var newSize = size * 2;
        var type = array1.GetValue(0)?.GetType();
        var array2 = Array.CreateInstance(type, newSize);

        for (int i = 0; i < size; i++) {
            array2.SetValue(array1.GetValue(i), i);
        }
        
        return array2;
    }

Joannejoannes