C#窗体计算器

使用窗体写的计算器小程序，不是十分完善，键盘输入只能输入数字键，其他需要换档键的键的输入没有搞懂，先发上来左右，以作留存。

界面截图

主要使用的是TextBox和button控件

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Data;
 using System.Drawing;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;

 namespace WindowsFormsApplication1
 {
     public partial class jisuanqi : Form
     {
         //存储上次点击了什么按钮，0代表什么都没点击，1代表点击了数字按钮，2代表点击了运算符
         private int prev = ;
         //存储中间结果
         private decimal zjjg = ;
         //记录上次按的什么运算符
         private string  preysf = "+";
         //记录上次按的是等号
         private int denghao = ;

         public jisuanqi()
         {
             InitializeComponent();
         }

         private void button1_Click(object sender, EventArgs e)
         {
             Button btn = sender as Button;

             //替换（如果下面文本框内容为0或者上次点击了运算符）
             if (prev ==  || TXTbottom.Text == "")
             {
                 TXTbottom.Text = btn.Text;
             }
             //追加（如果下面文本框内容不为0并且上次没有点击运算符）
             else
             {
                 TXTbottom.Text += btn.Text;
             }

             prev = ;

         }

         private void button4_Click(object sender, EventArgs e)
         {
             Button btn = sender as Button;

             //上次按了数字
             if (prev == )
             {
                 TXTTOP.Text += TXTbottom.Text + btn.Text;
                 switch (preysf)
                 {
                     case "+":
                         zjjg = zjjg + Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "-":
                         zjjg = zjjg - Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "*":
                         zjjg = zjjg * Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "/":
                         zjjg = zjjg / Convert.ToDecimal(TXTbottom.Text);
                         break;
                 }
                 TXTbottom.Text = zjjg.ToString();
             }
             //上次按了运算符///
             else
             {
                 string s = TXTTOP.Text;
                 if (s.Length >= )
                 {
                     s = s.Substring(, s.Length - );
                     s = s + btn.Text;
                 }
                 else
                 {
                     s = "";
                     s = s + btn.Text;
                 }
                 TXTTOP.Text = s;
             }
             //加号
             prev = ;
             preysf = btn.Text;
         }

         private void button15_Click(object sender, EventArgs e)
         {
             TXTTOP.Text = "";
             TXTbottom.Text = "";
             prev = ;
             zjjg = ;
             preysf = "+";
         }

         private void jisuanqi_KeyDown(object sender, KeyEventArgs e)
         {

             if (
                 e.KeyCode == Keys.D0 || e.KeyCode == Keys.NumPad0 ||
                 e.KeyCode == Keys.D1 || e.KeyCode == Keys.NumPad1 ||
                 e.KeyCode == Keys.D2 || e.KeyCode == Keys.NumPad2 ||
                 e.KeyCode == Keys.D3 || e.KeyCode == Keys.NumPad3 ||
                 e.KeyCode == Keys.D4 || e.KeyCode == Keys.NumPad4 ||
                 e.KeyCode == Keys.D5 || e.KeyCode == Keys.NumPad5 ||
                 e.KeyCode == Keys.D6 || e.KeyCode == Keys.NumPad6 ||
                 e.KeyCode == Keys.D7 || e.KeyCode == Keys.NumPad7 ||
                 e.KeyCode == Keys.D8 || e.KeyCode == Keys.NumPad8 ||
                 e.KeyCode == Keys.D9 || e.KeyCode == Keys.NumPad9
                 )
             {
                 if (prev ==  || TXTbottom.Text == "")
                 {
                     string a = e.KeyData.ToString();
                     TXTbottom.Text = a.Substring((a.Length-),);
                 }
                 else
                 {
                     string a = e.KeyData.ToString();
                     TXTbottom.Text += a.Substring((a.Length - ), ); 

                 }
                 prev = ;
             }
             //else if (
             //    //e.KeyCode == Keys.Add || e.KeyCode == Keys.Subtract || e.KeyCode == Keys.Multiply || e.KeyCode == Keys.Divide||
             //    e.KeyCode==Keys.Shift&&e.KeyCode==Keys.D8
             //    )
             //{
             //    MessageBox.Show(e.KeyData.ToString());
             //}
             else if(e.KeyCode==Keys.Back)
             {
                 string s = TXTbottom.Text;
                 if (s.Length >= )
                 {
                     s = s.Substring(, s.Length - );
                 }
                 else
                 {
                     s = "";
                 }
                 TXTbottom.Text = s;
             }
         }

         private void button17_Click(object sender, EventArgs e)
         {
             string s = TXTbottom.Text;
             if (s.Length >= )
             {
                 s = s.Substring(, s.Length - );
             }
             else
             {
                 s = "";
             }
             TXTbottom.Text = s;
         }

         private void button14_Click(object sender, EventArgs e)//等号
         {
             Button btn = sender as Button;

             //上次按了数字
             if (prev == )
             {
                 TXTTOP.Text += TXTbottom.Text + btn.Text;
                 switch (preysf)
                 {
                     case "+":
                         zjjg = zjjg + Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "-":
                         zjjg = zjjg - Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "*":
                         zjjg = zjjg * Convert.ToDecimal(TXTbottom.Text);
                         break;
                     case "/":
                         zjjg = zjjg / Convert.ToDecimal(TXTbottom.Text);
                         break;
                 }
                 TXTbottom.Text = zjjg.ToString();
             }
             //上次按了运算符///
             else
             {
                 string s = TXTTOP.Text;
                 if (s.Length >= )
                 {
                     s = s.Substring(, s.Length - );
                     s = s + btn.Text;
                 }
                 else
                 {
                     s = "";
                     s = s + btn.Text;
                 }
                 TXTbottom.Text = s;
             }
             //加号
             prev = ;
             preysf = btn.Text;
             TXTTOP.Text = "";
             zjjg = ;
             preysf = "+";
             denghao = ;
         }

         private void jisuanqi_Load(object sender, EventArgs e)
         {

         }
     }
 }

程序冗余还是有点多，目前没有想要要怎么精简