For testing purpose, I need to generate some random string (contains only letters without any other special characters) with customized length. So I extends the C# built-in Random class like this:

#region Random's Extension
public static class RandomExtension
{
    /// 

    /// Returns a random character between the start and end characters specified
    /// 
    /// 
    /// The start of the range that the next random character will be generated from
    /// The end of the range that the next random character will be generated from
    /// A character whose ASCII code greater than or equal to the start's and less than or equal to the end's
    public static char NextChar(this Random rnd, char start = 'a', char end = 'z')
    {
        int startCode = (int)start;
        int endCode = (int)end + 1;
        if (startCode <= endCode)
        {
            int code = rnd.Next(startCode, endCode);
            return (char)code;
        }
        else
        {
            throw new ArgumentException("The 'start' character can NOT be greater than the 'end' charcater", "start");
        }
    }

    /// 

    /// Returns a random character among a set of specified characters
    /// 
    /// 
    /// A set of the characters that the new random character will be generated from
    /// A character from the specified character set
    public static char NextChar(this Random rnd, char[] candidates)
    {
        if (candidates.Length > 0)
            return candidates[rnd.Next(0, candidates.Length)];
        else
            throw new ArgumentException("Must specify at least 1 character in the array (char[] candidates).", "candidates");
    }

    /// 

    /// Returns a random letter character ({'a' - 'z'} + {'A' - 'Z'})
    /// 
    /// 
    /// A character of the 26 English letters ignoring case.
    public static char NextLetter(this Random rnd)
    {
        return rnd.NextChar(new char[] { rnd.NextChar('a', 'z'), rnd.NextChar('A', 'Z') });
    }

    /// 

    /// Returns a random letter string (a string contains only letters, no other special characters) with customized length
    /// 
    /// 
    /// The length that the random string will be in
    /// A string contains only letters.
    public static string NextLetterString(this Random rnd, int length = 10)
    {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < length; i++)
        {
            sb.Append(rnd.NextLetter());
        }
        return sb.ToString();
    }
}
#endregion