CopyTo

This C# method copies List elements to an array. It provides a way to do this in one declarative function call—this keeps code simpler.

Method notes

When using the CopyTo() method, we must make sure the target array is properly allocated. And the element types must match.

Complex example

To begin, we have a List of ints and we call its CopyTo method with an index argument. Copying begins in the array at the argument index.

using System;
using System.Collections.Generic;

var list = new List<int>() { 10, 20, 30 };
int[] array = new int[5];
// Use CopyTo to copy into an array starting at a certain index.
// .. The entire list is copied.
list.CopyTo(array, 2);
Console.WriteLine("LIST:  " + string.Join(",", list));
Console.WriteLine("ARRAY: " + string.Join(",", array));
LIST:  10,20,30
ARRAY: 0,0,10,20,30

Simple example

This program creates a List with three elements in it: the values 5, 6 and 7. Next an int array is allocated. It has a length equal to the Count of the List.

using System;
using System.Collections.Generic;

// Create a list with 3 elements.
var list = new List<int>() { 5, 6, 7 };

// Create an array with length of 3.
int[] array = new int[list.Count];

// Copy the list to the array.
list.CopyTo(array);

// Display.
Console.WriteLine(array[0]);
Console.WriteLine(array[1]);
Console.WriteLine(array[2]);
5
6
7

Internals

CopyTo() calls Array.Copy on the internal array inside the List. Examining the secrets inside .NET implementations helps us learn good practices.

Summary

The List CopyTo method provides a way to copy all the elements of a List to a compatible and allocated array. The array must have adequate space for all the elements.

Elements, review

The type of the elements in the array must be equivalent to the type of the List elements. Our code must have a correct target array.