Array.Copy

This C# method copies elements from one array to another. It has some complexities. This operation can result in certain exceptions.

Element types

The type of elements—in both the target and source arrays—is important. Array.Copy is a static method—we reference it directly.

First example

Here we use the Array.Copy method overload that copies one source array to a destination array. Both arrays must have at least the length specified in the third parameter.

using System;

// Step 1: instantiate the source array.
int[] source = new int[5];
source[0] = 1;
source[1] = 2;
source[2] = 3;
source[3] = 4;
source[4] = 5;

// Step 2: instantiate and allocate the target array.
int[] target = new int[5];

// Step 3: copy the source to the target.
Array.Copy(source, target, 5);

// Step 4: display the target array.
foreach (int value in target)
{
    Console.WriteLine(value);
}
1
2
3
4
5

Range example

Here we copy one range in an array to another range in the second array. This style of code can cause exceptions. We have to be careful with checking the array bounds.

using System;

// Source array.
int[] source = new int[5];
source[0] = 5;
source[1] = 4;
source[2] = 3;
source[3] = 2;
source[4] = 1;

// Instantiate the target.
int[] target = new int[3];

// Copy first 3 elements in source to target.
Array.Copy(source, 0, target, 0, 3);

// Display the result.
foreach (int value in target)
{
    Console.WriteLine(value);
}
5
4
3

Array.ConstrainedCopy

This method validates arrays before it copies them. The regular Array.Copy method will silently copy a byte array to an int array.

using System;

byte[] original = new byte[10];
original[0] = 1;

int[] destination = new int[10];

// This will work if you call Array.Copy instead.
Array.ConstrainedCopy(original, 0, destination, 0, original.Length);
Unhandled Exception:
System.ArrayTypeMismatchException: Array.ConstrainedCopy will only work
on array types that are provably compatible, without any form of boxing,
unboxing, widening,or casting of each array element.
Change the array types....

Performance

The Array.Copy method is fast for most uses. It is simpler than manually copying all the elements. Array.Copy can be optimized in .NET.

using System;
using System.Diagnostics;

const int _max = 10000000;
// Set up benchmark.
var data = new byte[200];
for (int i = 50; i < 75; i++)
{
    data[i] = (byte)i;
}
var buffer = new byte[100];

var s1 = Stopwatch.StartNew();
// Version 1: use Array.Copy.
for (int i = 0; i < _max; i++)
{
    Array.Copy(data, 50, buffer, 0, 25);
}
s1.Stop();
var s2 = Stopwatch.StartNew();
// Version 2: use for-loop to copy.
for (int i = 0; i < _max; i++)
{
    for (int z = 0; z < 25; z++)
    {
        buffer[z] = data[z + 50];
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
 7.17 ns    Array.Copy
13.86 ns    for

Copy, exceptions

Here we look at Array.Copy exceptions. First, you cannot copy from or to a null reference array. This raises the ArgumentNullException.

List

You can use the List generic class instead of Array to copy collections of elements. With the List class, you can copy into a new List simply by using the constructor.

A summary

Array.Copy can copy entire arrays or just sections of arrays. You must remember to allocate your arrays with adequate Length before calling Array.Copy.