XO.cs

/// <summary>
        /// Check to see if a string has the same amount of 'x's and 'o's.
        /// The method must return a boolean and be case insensitive.
        /// </summary>
        /// <param name="input">The string can contain any char.</param>
        /// <returns></returns>
        public static bool XO(string input)
        {
            int xCounter = 0;
            int oCounter = 0;

            foreach (char item in input)
            {
                if ("x" == item.ToString().ToLower())
                {
                    xCounter++;
                }
                else if ("o" == item.ToString().ToLower())
                {
                    oCounter++;
                }
            }
            if (xCounter == oCounter)
            {
                return true;
            }
            else
            {
                return false;
            }
        }

XO2.cs

        /// <summary>
        /// Same as XO, just in one line.
        /// </summary>
        /// <param name="input"></param>
        /// <returns></returns>
        public static bool XO2(string input)
        {
            return input.ToLower().Count(i => i == 'x') == input.ToLower().Count(i => i == 'o');
        }