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copied!<p><s>The <code>string</code> class is immutable (although a reference type), hence all its static methods are designed to return a <em>new</em> <code>string</code> variable. Calling <code>someString.Replace</code> without assigning it to anything will not have any effect in your program.</s> - Seems like you fixed this problem. </p> <p>The main issue with your suggested algorithm is that it repeatedly assigning many new <code>string</code> variables, potentially causing a big performance hit. LINQ doesn't really help things here. (I doesn't make the code significantly shorter and certainly not any more readable, in my opinion.)</p> <p>Try the following extension method. The key is the use of <code>StringBuilder</code>, which means only one block of memory is assigned for the result during execution.</p> <pre><code>private static readonly HashSet<char> badChars = new HashSet<char> { '!', '@', '#', '$', '%', '_' }; public static string CleanString(this string str) { var result = new StringBuilder(str.Length); for (int i = 0; i < str.Length; i++) { if (!badChars.Contains(str[i])) result.Append(str[i]); } return result.ToString(); } </code></pre> <p>This algorithm also makes use of the .NET 3.5 'HashSet' class to give <code>O(1)</code> look up time for detecting a bad char. This makes the overall algorithm <code>O(n)</code> rather than the <code>O(nm)</code> of your posted one (<code>m</code> being the number of bad chars); it also is lot a better with memory usage, as explained above.</p>

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