CopyTo

This C# method takes string characters and puts them into an array. It copies a group of characters from one source string into a character array.

Optimized code

This .NET string method provides optimized low-level code. When we need to copy chars from a string, CopyTo is a good solution.

An example

CopyTo() must be called on an instance of a string object. String objects in C# can be represented by string literals. So we can call CopyTo on a literal as well.

using System;

string value1 = "abcd";
char[] array1 = new char[3];
// Copy the final 3 characters to the array.
value1.CopyTo(1, array1, 0, 3);
// Print the array we copied to.
Console.WriteLine($"RESULT LENGTH = {array1.Length}");
Console.WriteLine(array1);
RESULT LENGTH = 3
bcd

A benchmark

I wanted to know whether using CopyTo is faster than a for-loop on a short string. Should we use CopyTo in performance-critical methods?

using System;
using System.Diagnostics;

const int _max = 100000000;
char[] values = new char[100];
string temp = "0123456789";
// Version 1: use CopyTo.
// Version 2: use for-loop.
var s1 = Stopwatch.StartNew();
if (true)
{
    for (int i = 0; i < _max; i++)
    {
        temp.CopyTo(0, values, 0, temp.Length);
    }
}
else
{
    for (int i = 0; i < _max; i++)
    {
        for (int j = 0; j < temp.Length; j++)
        {
            values[j] = temp[j];
        }
    }
}
s1.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
 7.38 ns, CopyTo
12.39 ns, For-loop

Substring

In most .NET programs, the CopyTo method is not necessary. Instead, your programs will often use Substring to copy one range of characters to another.

Summary

CopyTo() allows you to copy ranges of characters from a source string into target arrays. It is often not needed, but can replace a for-loop to copy chars.