Efficient Replace Special char method

RemoveSpecialCharacters

public static string RemoveSpecialCharacters(string str)
{
    int idx = 0;
    char[] chars = new char[str.Length];
    foreach (char c in str)
    {
        if ((c >= '0' && c <= '9')
            || (c >= 'A' && c <= 'Z') 
            || (c >= 'a' && c <= 'z') 
            || (c == '.') || (c == '_'))
        {
            chars[idx++] = c;
        }
    }
    return new string(chars, 0, idx);
}

See
http://sixlettervariable.blogspot.com/search/label/c%23

http://stackoverflow.com/questions/1120198/most-efficient-way-to-remove-special-characters-from-string/1120407#1120407