Calculator

using System.Text;
//123 + 456 = 579
namespace ConsoleCalculator
{
    class Program
    {
        static void Main()
        {
            Console.WriteLine("=== Simple Calculator ===");
            Console.WriteLine("Type an expression and press '=' to execute.");
            Console.WriteLine("Press Enter (empty line) to exit.\n");

            var expression = new StringBuilder();

            while (true)
            {
                var keyInfo = Console.ReadKey(intercept: true);
                char keyChar = keyInfo.KeyChar;

                if (keyInfo.Key == ConsoleKey.Enter)
                {
                    if (expression.Length == 0)
                    {
                        Console.WriteLine("\nGoodbye!");
                        break;
                    }
                    else
                    {
                        Console.WriteLine();
                        continue;
                    }
                }

                if (keyInfo.Key == ConsoleKey.Backspace)
                {
                    if (expression.Length > 0)
                        expression.Remove(expression.Length - 1, 1);
                }
                else if (keyChar == '=')
                {
                    // Execute the current expression
                    Console.WriteLine();
                    Console.WriteLine(CalculateSimpleExpression(expression.ToString()));
                    break;
                }
                else
                {
                    // Append valid characters only
                    if (char.IsDigit(keyChar) || "+-*/x().".Contains(keyChar))
                        expression.Append(keyChar);
                }

                Console.Clear();
                Console.Write(expression.ToString());
            }
        }

        static double CalculateSimpleExpression(string expression)
        {
            if (string.IsNullOrWhiteSpace(expression))
                return 0;

            var numbers = new List<double>();
            var operators = new List<char>();
            var current = new StringBuilder();

            foreach (char c in expression)
            {
                if (char.IsDigit(c) || c == '.')
                {
                    current.Append(c);
                }
                else if ("+-*/".Contains(c))
                {
                    if (current.Length > 0)
                    {
                        numbers.Add(double.Parse(current.ToString()));
                        current.Clear();
                    }
                    operators.Add(c);
                }
            }

            if (current.Length > 0)
                numbers.Add(double.Parse(current.ToString()));

            if (numbers.Count == 0)
                return 0;

            // Perform operations left to right
            double result = numbers[0];
            for (int i = 0; i < operators.Count; i++)
            {
                double next = numbers[i + 1];
                char op = operators[i];

                result = op switch
                {
                    '+' => result + next,
                    '-' => result - next,
                    '*' => result * next,
                    '/' => next != 0 ? result / next : double.NaN,
                    _ => result
                };
            }

            return result;
        }
    }
}