Fastest way to remove white spaces in string

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I would build a custom extension method using StringBuilder, like:

public static string ExceptChars(this string str, IEnumerable<char> toExclude)
{
    StringBuilder sb = new StringBuilder(str.Length);
    for (int i = 0; i < str.Length; i++)
    {
        char c = str[i];
        if (!toExclude.Contains(c))
            sb.Append(c);
    }
    return sb.ToString();
}

Usage:

var str = s.ExceptChars(new[] { ' ', '\t', '\n', '\r' });

or to be even faster:

var str = s.ExceptChars(new HashSet<char>(new[] { ' ', '\t', '\n', '\r' }));

With the hashset version, a string of 11 millions of chars takes less than 700 ms (and I’m in debug mode)

EDIT :

Previous code is generic and allows to exclude any char, but if you want to remove just blanks in the fastest possible way you can use:

public static string ExceptBlanks(this string str)
{
    StringBuilder sb = new StringBuilder(str.Length);
    for (int i = 0; i < str.Length; i++)
    {
        char c = str[i];
        switch (c)
        {
            case '\r':
            case '\n':
            case '\t':
            case ' ':
                continue;
            default:
                sb.Append(c);
                break;
        }
    }
    return sb.ToString();
}

EDIT 2 :

as correctly pointed out in the comments, the correct way to remove all the blanks is using char.IsWhiteSpace method :

public static string ExceptBlanks(this string str)
{
    StringBuilder sb = new StringBuilder(str.Length);
    for (int i = 0; i < str.Length; i++)
    {
        char c = str[i];
        if(!char.IsWhiteSpace(c))
            sb.Append(c);
    }
    return sb.ToString();
}

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