Generate Random String Containing Numbers and Strings Only

/// <summary>
        /// Generates random string of length provided.
        /// The string will contain only a-z, A-z and 0-9 characters
        /// </summary>
        /// <param name="length"></param>
        /// <returns></returns>
        public string RandomString(int length)
        {
            Random rand = new Random();

            char randomNumber()
            {
                return (char)rand.Next(48, 57);
            }

            char randomUpperCase()
            {
                return (char)rand.Next(65, 90);
            }
            char randomLowerCase()
            {
                return (char)rand.Next(97, 122);
            }

            string str = "";
            for (int i = 0; i < length; i++)
            {
                switch (rand.Next(0, 2))
                {
                    case 0: str += randomNumber().ToString(); break;
                    case 1: str += randomLowerCase().ToString(); break;
                    case 2: str += randomUpperCase().ToString(); break;
                }
            }
            return str;
        }