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How to Toggle String Case in .NET

By , 22 Feb 2011
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Following two Toggle Case Algorithms, implemented as "pure" .NET solution (no "unsafe" coding technique, all managed code) demonstrate the best performance, tested against a variety of text strings, containing: ASCII, Unicode, all Low case, all Upper case, long numeric strings (refer to the test site:[^])
1. Ayoola-Bell Toggle Case Algorithm
(Originally posted by Henry Ayoola, modified by Alex Bell)

/// <summary>Ayoola-Bell Toggle Case Algorithm</summary>
/// <param name="s">string</param>
/// <returns>string</returns>
public static string ToggleCase_Ayoola_Bell(string s)
    char[] chs = s.ToCharArray();
    for (int i = s.Length - 1; i >= 0; i--)
        char ch = chs[i];
        if (char.IsLetter(ch))
            char foo = (char)(ch & ~0x20);
            if ((foo >= 0x41 && foo <= 0x5a) ||
                (foo >= 0xc0 && foo <= 0xde && foo != 0xd7))
                chs[i] = (char)(ch ^ 0x20);
            else if ((foo == 0xdf || ch > 0xff))
                chs[i] = char.IsLower(ch) ?
                         char.ToUpper(ch) :
    return (new String(chs));
The above algorithm is the fastest .NET solution; it implements the important char.IsLetter check prior to processing the text string, thus giving tremendous performance boost when converting the strings containing significant amount of non-letter characters.
Same technique is applied to the algorithm by nedel resulted in elegant, rather simple yet effective .NET solution, trailing the first one in terms of performance: the difference is almost insignificant in case of processing the Unicode strings.
2. Bell-Nedel Toggle Case Algorithm
(Originally posted by Nedel, modified by Alex Bell)

/// <summary>Bell_Nedel Toggle Case Algorithm</summary>
/// <param name="s">string</param>
/// <returns>string</returns>
protected string ToggleCase_Bell_Nedel(string s)
    char[] charArr = s.ToCharArray();
    for (int i = 0; i < charArr.Length; ++i) {
        if (char.IsLetter(charArr[i]))
            charArr[i] = char.IsLower(charArr[i]) ?
                         char.ToUpper(charArr[i]) :
    return (new String(charArr));
Note: In terms of performance, the absolute speed champion in this "League of Extraordinary Efficient Toggle Case Algorithms" Smile | :) is the solution provided by Michael Hansen; be aware, that his solution is using "unsafe" coding technique, going beyond the boundaries of .NET managed code.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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President Infosoft International Inc
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Dr. A. Bell has 20+ years of Software and Electrical Engineering experience. He is Win/Web veteran, published 200+ articles and authored 37 inventions, currently focused on: Windows 7/8, HTML5, CSS3, jQuery, SQL, .NET, ASP.NET, WPF, C#, Speech Technology and Mobile apps. He's been among recent App submission winners (The Windows 8* & Ultrabook™ App Innovation Contest 2012). Sample pubs:
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Comments and Discussions

GeneralYou seem to have modified my code without fully understandin... PinmemberHenry.Ayoola8-Mar-11 2:17 
You seem to have modified my code without fully understanding it (although I admit that I messed up the bracketing). The point of the check (foo == 0xdf || ch > 0xff) was to avoid calling char.IsLetter on characters which were already known not to be letters. If you're going to pull that check out then you can ditch the inner check and just have a plain else clause.

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