程序代码：

using System;
using System.Collections.Generic;


namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            string expression;
            Console.Write("输入算式：");
            expression = Console.ReadLine();
            Console.WriteLine(new Calculator().Calculate(expression));
        }


        public class Calculator
        {
            public Calculator()
            {
                _OptStack = new Stack<char>();
                _SuffixStack = new Stack<float>();
            }
            private Stack<char> _OptStack;
            private Stack<float> _SuffixStack;
            public float Calculate(string expression)
            {
                string lastNum = string.Empty;
                for (int i = 0; i < expression.Length; i++)
                {
                    if (char.IsNumber(expression[i]) || expression[i].Equals('.'))
                    {
                        lastNum += expression[i];
                    }
                    else
                    {
                        if (lastNum != string.Empty)
                        {
                            Merger(float.Parse(lastNum));
                            lastNum = string.Empty;
                        }
                        AddOpt(expression[i]);
                    }
                }
                if (lastNum != string.Empty)
                {
                    Merger(float.Parse(lastNum));
                }
                while (_OptStack.Count > 0)
                {
                    Merger(_OptStack.Pop());
                }
                return _SuffixStack.Pop();
            }
            private void AddOpt(char opt)
            {
                if (_OptStack.Count == 0)
                {
                    _OptStack.Push(opt);
                    return;
                }
                if (opt.Equals(')'))
                {
                    while (!_OptStack.Peek().Equals('('))
                    {
                        Merger(_OptStack.Pop());
                    }
                    _OptStack.Pop();
                    return;
                }
                char tempOpt = _OptStack.Peek();
                if ((opt.Equals('-') || opt.Equals('+')) &&
                    (tempOpt.Equals('*') || tempOpt.Equals('/')))
                {
                    while (_OptStack.Count > 0)
                    {
                        Merger(_OptStack.Pop());
                    }
                }
                _OptStack.Push(opt);
            }
            private void Merger(float exp)
            {
                _SuffixStack.Push(exp);
            }
            private void Merger(char exp)
            {
                float num1 = _SuffixStack.Pop();
                float num2 = _SuffixStack.Pop();
                float result = 0;
                switch (exp)
                {
                    case '+':
                        result = num2 + num1;
                        break;
                    case '-':
                        result = num2 - num1;
                        break;
                    case '*':
                        result = num2 * num1;
                        break;
                    case '/':
                        result = num2 / num1;
                        break;
                }
                _SuffixStack.Push(result);
            }
        }
    }
}