Efficient way to remove ALL whitespace from String?

This is fastest way I know of, even though you said you didn't want to use regular expressions:

Regex.Replace(XML, @"\s+", "");

Crediting @hypehuman in the comments, if you plan to do this more than once, create and store a Regex instance. This will save the overhead of constructing it every time, which is more expensive than you might think.

private static readonly Regex sWhitespace = new Regex(@"\s+");
public static string ReplaceWhitespace(string input, string replacement) 
{
    return sWhitespace.Replace(input, replacement);
}

Try the replace method of the string in C#.

XML.Replace(" ", string.Empty);

I have an alternative way without regexp, and it seems to perform pretty good. It is a continuation on Brandon Moretz answer:

 public static string RemoveWhitespace(this string input)
 {
    return new string(input.ToCharArray()
        .Where(c => !Char.IsWhiteSpace(c))
        .ToArray());
 }

I tested it in a simple unit test:

[Test]
[TestCase("123 123 1adc \n 222", "1231231adc222")]
public void RemoveWhiteSpace1(string input, string expected)
{
    string s = null;
    for (int i = 0; i < 1000000; i++)
    {
        s = input.RemoveWhitespace();
    }
    Assert.AreEqual(expected, s);
}

[Test]
[TestCase("123 123 1adc \n 222", "1231231adc222")]
public void RemoveWhiteSpace2(string input, string expected)
{
    string s = null;
    for (int i = 0; i < 1000000; i++)
    {
        s = Regex.Replace(input, @"\s+", "");
    }
    Assert.AreEqual(expected, s);
}

For 1,000,000 attempts the first option (without regexp) runs in less then a second (700 ms on my machine), and the second takes 3.5 seconds.

My solution is to use Split and Join and it is surprisingly fast, in fact the fastest of the top answers here.

str = string.Join("", str.Split(default(string[]), StringSplitOptions.RemoveEmptyEntries));

Timings for 10,000 loop on simple string with whitespace inc new lines and tabs

• split/join = 60 milliseconds
• linq chararray = 94 milliseconds
• regex = 437 milliseconds

Improve this by wrapping it up in method to give it meaning, and also make it an extension method while we are at it ...

public static string RemoveWhitespace(this string str) {
    return string.Join("", str.Split(default(string[]), StringSplitOptions.RemoveEmptyEntries));
}