Because String is a sealed class you cannot inherit from it and add to it. However, C# has a wonderful feature called extension methods.

Extension methods enable you to “add” methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type. [1]

Here is a list of extension methods I have used for the String class.

If you know of any more cool extension methods, please comment.

using System;
using System.Runtime.InteropServices;
using System.Text.RegularExpressions;
using System.Security;

namespace LANDesk.Install.Common.Extenders
{
    /// <summary>
    /// This class adds methods to String that are useful
    /// </summary>
    public static class StringExtender
    {
        #region DLL Imports
        [DllImport("ldmsinst.dll", CharSet = CharSet.Ansi, EntryPoint = "Decrypt", CallingConvention = CallingConvention.Cdecl)]
        public static extern void Decrypt(int pwdLen, byte[] pwd);

        [DllImport("ldmsinst.dll", CharSet = CharSet.Ansi, EntryPoint = "Encrypt", CallingConvention = CallingConvention.Cdecl)]
        public static extern void Encrypt(int length, byte[] data);
        #endregion

        /// <summary>
        /// This adds a function to a string object that will convert the string
        /// to a SecureString.
        /// </summary>
        /// <param name="password">The string to convert to a secure string.</param>
        /// <returns>SecureString</returns>
        public static SecureString ConvertToSecureString(this string password)
        {
            SecureString ss = new SecureString();
            if (password == null)
                throw new ArgumentNullException("password");

            foreach (Char c in password)
            {
                ss.AppendChar(c);
            }
            return ss;
        }

        /// <summary>
        /// Checks if the string contains any of a list of characters
        /// entered as a string.
        /// </summary>
        /// <param name="password">Any string.</param>
        /// <param name="listofcharacters">Any string representing a list of characters.</param>
        /// <returns>bool</returns>
        public static bool ContainsAnyOf(this string password, char[] listofcharacters)
        {
            foreach (char c in listofcharacters)
            {
                if (password.Contains(c.ToString()))
                    return true;
            }
            return false;
        }

        /// <summary>
        /// Checks if the string contains any of a list of characters
        /// entered as a string.
        /// </summary>
        /// <param name="password">Any string.</param>
        /// <param name="listofcharacters">Any string representing a list of characters.</param>
        /// <returns>bool</returns>
        public static bool ContainsAnyOf(this string password, string listofcharacters)
        {
            return password.ContainsAnyOf(listofcharacters.ToCharArray());
        }

        /// <summary>
        /// Checks if the string has a space.
        /// </summary>
        /// <param name="txt">Any string.</param>
        /// <returns>bool</returns>
        public static bool HasSpaces(string txt)
        {
            if (txt.Contains(" "))
            {
                return true;
            }
            return false;
        }

        /// <summary>
        /// To be a strong password, Must have 3 out of 4 of the following conditions:
        /// Uppercase, lowercase, numberic, symbol
        /// </summary>
        /// </summary>
        /// <param name="password"></param>
        /// <returns></returns>
        public static bool IsPasswordStrong(this string password)
        {
            int numTypes = 0;

            if (HasUppercaseCharacter(password) == true)
            {
                numTypes = numTypes + 1;
            }
            if (HasLowercaseCharacter(password) == true)
            {
                numTypes = numTypes + 1;
            }
            if (HasNonAlphaNumeric(password) == true)
            {
                numTypes = numTypes + 1;
            }
            if (HasDigit(password) == true)
            {
                numTypes = numTypes + 1;
            }

            if (numTypes > 2)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        public static bool HasUppercaseCharacter(string str)
        {
            if (str == str.ToLower())
                return false;
            return true;
        }

        public static bool HasLowercaseCharacter(string str)
        {
            if (str == str.ToUpper())
                return false;
            return true;
        }

        public static bool HasDigit(string str)
        {
            Regex regexNums = new Regex("[0-9]");
            return regexNums.IsMatch(str);
        }

        public static bool HasNonAlphaNumeric(string str)
        {
            string newString = str;

            Regex regexNums = new Regex("[0-9]");
            Regex regexAlphas = new Regex("[a-z]", RegexOptions.IgnoreCase);

            newString = regexNums.Replace(newString, "");
            newString = regexAlphas.Replace(newString, "");

            return (newString.Length > 0);
        }

        /// <summary>
        /// This adds a function to a string object that RC4 Encrypts a password.
        /// </summary>
        /// <param name="password">A password as a string.</param>
        /// <returns>A byte[] representation of an RC4 password.</returns>
        public static byte[] Encrypt(this string password)
        {
            try
            {
                // Encode the password
                System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
                byte[] encryptedPwd = encoding.GetBytes(password);

                // encrypt password
                Encrypt(encryptedPwd.Length, encryptedPwd);
                return encryptedPwd;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        /// <summary>
        /// This adds a function to a byte[] object that decrypts an RC4 password.
        /// and returns the password as a string.
        /// </summary>
        /// <param name="encryptedPwd">A byte[] representation of an RC4 password.</param>
        /// <returns>The password as a string.</returns>
        static public string Decrypt(this byte[] encryptedPwd)
        {
            string password = string.Empty;
            try
            {
                    // decrypt password
                    Decrypt(encryptedPwd.Length, encryptedPwd);

                    // convert from byte array to string
                    password = System.Text.ASCIIEncoding.Default.GetString(encryptedPwd);
                    password = password.Replace("\0", string.Empty);
            }
            catch (Exception ex)
            {
                throw ex;
            }
            return password;
        }

        public static string FirstLetterUpperCase(this string inString)
        {
            if (string.IsNullOrEmpty(inString))
                return string.Empty;

            string newString = inString.Substring(0, 1).ToUpper();
            newString += inString.Substring(1, inString.Length - 1);
            return newString;
        }

        public static string LimitTo(this string data, int length)
        {
            return data.Length < length ? data : data.Substring(0, length);
        }
    }
}