C#+Windows Formで作るダメな電卓例(改善版) ref: http://qiita.com/yukkuribemani/items/ddf25a9534b5e58c6ad9

calc.cs

using System;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace calc
{
    public partial class Form1 : Form
    {
        private bool Form_Write_Permit = true;

        public Form1()
        {
            InitializeComponent();
        }

        private void btnNumber_Click(object sender, EventArgs e)
        {
            if (Form_Write_Permit == false)
            {
                return;
            }
            textBoxCalc.Text += ((Button)sender).Text;
        }

        private void Op_Click(object sender, EventArgs e)
        {
            if (input_check() != 1)
            {
                return;
            }
            textBoxCalc.Text += ((Button)sender).Text;
        }

        private void point_Click(object sender, EventArgs e)
        {
            if(input_check() != 1)
            {
                return;
            }
            textBoxCalc.Text += '.';
        }

        private void Clear_Click(object sender, EventArgs e)//式の初期化
        {
            textBoxCalc.Text = String.Empty;
            Form_Write_Permit = true;
        }

        private void eq_Click(object sender, EventArgs e)//式の計算を行う
        {
            char Operator = ' ';
            string Form_Num_Copy = string.Empty;
            double Form_Check;
            double Left_Value = 0.0; //演算子で式を分けた時の左と右の数値
            double Right_Value = 0.0;
            bool First_Found = true;　// 初回の計算かどうか判定

            //  フォーム内に数値以外もしくは空のときに"="を押しても何もしない
            if (double.TryParse(textBoxCalc.Text, out Form_Check) || textBoxCalc.Text == "")
            {
                    return;
            }

            for (int i = 0; i < textBoxCalc.Text.Length;i++) //フォームから1文字ずつ読みだす
            {
                char Char_Temp = textBoxCalc.Text[i];
                if (Char.IsDigit(Char_Temp) == true || Char_Temp == '.') //数値もしくは小数点の場合
                {
                    Form_Num_Copy += Char_Temp;    //数値のコピーをとっておく
                    if(i == textBoxCalc.Text.Length - 1)//数値挿入時点で式が終了するか調べる
                    {
                        Right_Value = double.Parse(Form_Num_Copy);
                        Left_Value = calc(Left_Value, Right_Value, Operator);
                        Form_Num_Copy = string.Empty;
                    }
                }
                else if (Char_Temp == '+' || Char_Temp == '-' || Char_Temp == '*' || Char_Temp == '/')//いずれかの演算子の場合
                {
                    if(First_Found == true)//初めて演算子を見つけたとき
                    {
                        Left_Value = double.Parse(Form_Num_Copy);//1回目のLeft_Valueの代入
                        Form_Num_Copy = string.Empty;
                        First_Found = false;
                    }
                    else  //  2回目以降の演算
                    {
                        Right_Value =  double.Parse(Form_Num_Copy);
                        Left_Value = calc(Left_Value, Right_Value, Operator);
                        Form_Num_Copy = string.Empty;
                    }
                    Operator = Char_Temp;
                }               
            }
            textBoxCalc.Text = Left_Value.ToString();
            Form_Write_Permit = false;//計算後に数値を入力しても受け付けないようにする
        }

        private double calc(double x, double y,char op)
        {
            double ans = 0.0;
            switch(op)
            {
                case '+':
                    ans =  x + y;
                    break;
                case '-':
                    ans = x - y;
                    break;
                case '*':
                    ans = x * y;
                    break;
                case '/':
                    ans = x / y;
                    break;
            }
            return ans;
        }

        // 入力内容のチェック
        private int input_check()
        {
            if (textBoxCalc.Text == string.Empty)
            {
                return -1;
            }
            else if (textBoxCalc.Text[textBoxCalc.Text.Length - 1] == '+' || textBoxCalc.Text[textBoxCalc.Text.Length - 1] == '-' ||
                textBoxCalc.Text[textBoxCalc.Text.Length - 1] == '*' || textBoxCalc.Text[textBoxCalc.Text.Length - 1] == '/' ||
                textBoxCalc.Text[textBoxCalc.Text.Length - 1] == '.')
            {
                return -1;
            }
            return 1;
        }
    }
}