Reverse string

A string in C# can be reversed. A reversal method can be implemented in several ways. Some ways are faster than others.

Some methods use more lines of code—they are more complex to understand. We review and time some string reversal methods in .NET.

Example

This solution requires just 3 lines of code. The method shown here is based on ToCharArray, which can help performance. It is static because it stores no state.

using System;

class Program
{
    static string ReverseString(string s)
    {
        // Convert to char array, then call Array.Reverse.
        // ... Finally use string constructor on array.
        char[] array = s.ToCharArray();
        Array.Reverse(array);
        return new string(array);
    }

    static void Main()
    {
        Console.WriteLine(ReverseString("BIRD"));
        Console.WriteLine(ReverseString("CAT"));
    }
}
DRIB
TAC

Benchmark, string reversal

Suppose your program must reverse strings millions or billions of times. Can we use some simple logic to optimize string reversal? Here we test 2 methods.

using System;
using System.Diagnostics;

class Program
{
    public static string ReverseString(string s)
    {
        char[] array = s.ToCharArray();
        Array.Reverse(array);
        return new string(array);
    }

    public static string ReverseStringDirect(string s)
    {
        char[] array = new char[s.Length];
        int forward = 0;
        for (int i = s.Length - 1; i >= 0; i--)
        {
            array[forward++] = s[i];
        }
        return new string(array);
    }

    static void Main()
    {
        int sum = 0;
        const int _max = 10000000;
        Console.WriteLine(ReverseString("TEST"));
        Console.WriteLine(ReverseStringDirect("TEST"));

        // Version 1: reverse with ToCharArray.
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += ReverseString("PROGRAMMINGISFUN").Length;
        }
        s1.Stop();

        // Version 2: reverse with iteration.
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += ReverseStringDirect("PROGRAMMINGISFUN").Length;
        }
        s2.Stop();

        Console.WriteLine(s1.Elapsed.TotalMilliseconds);
        Console.WriteLine(s2.Elapsed.TotalMilliseconds);
    }
}
TSET
TSET
210.2514 ms    ReverseString
392.5179 ms    ReverseStringDirect

Performance notes

For optimal performance, use caching. Also consider just iterating through a string backwards instead of ever creating a string.

We saw a method that receives a string and returns the string with its characters in the reverse order. It is one of many examples of using char arrays for string manipulation.