求助，C#计算器问题

为什么我这个计算器在输完第二个数字的时候，结果却错误，比如11-11=10，真心求指教，代码如下；
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;namespace 计算
{    public partial class Form1 : Form
    {
        public enum CalcuType
        {
            None,
            JIA,
            JIAN,
            CHENG,
            CHU,
            KAIFANG,
            CHUFANG,
        }
        private double ? ValueF = null ;
        private double ? ValueL = null ;
        private CalcuType Calculatetype = CalcuType.None;
        private bool isNew = false;
        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            btn0.Click +=new EventHandler(btn_Click);
            btn1.Click += new EventHandler(btn_Click);
            btn2.Click += new EventHandler(btn_Click);
            btn3.Click += new EventHandler(btn_Click);
            btn4.Click += new EventHandler(btn_Click);
            btn5.Click += new EventHandler(btn_Click);
            btn6.Click += new EventHandler(btn_Click);
            btn7.Click += new EventHandler(btn_Click);
            btn8.Click += new EventHandler(btn_Click);
            btn9.Click += new EventHandler(btn_Click);
        }
        private void btn_Click(object sender, EventArgs e)//是不是这里面出问题啦？
        {
            Button btn = (Button)sender;
            string NumberStr = btnShow.Text;
            if (this.isNew)
            {
                NumberStr  = btn.Text;
                this.ValueL = double.Parse(NumberStr);
            }
            else
            {
                
                if (new string[] { "0", "0.", "-0", "-0." }.Contains(NumberStr ))
                {
                    NumberStr = "";
                }
                NumberStr += btn.Text;
                this.ValueF = double.Parse(NumberStr);
            }                        
             this.btnShow.Text = NumberStr ;
             this.isNew = false;
        }             
        private void btnResult_Click(object sender, EventArgs e)
        {
            switch (Calculatetype)
            {
                case CalcuType.JIA:
                    this.btnShow.Text = (ValueF + ValueL).ToString ();
                    break;
                case CalcuType.JIAN:
                    this.btnShow.Text = (ValueF - ValueL).ToString();
                    break;
                case CalcuType.CHENG:
                    this.btnShow.Text = (ValueF * ValueL).ToString();
                    break;
                case CalcuType.CHU:
                    this.btnShow.Text = (ValueF / ValueL).ToString();
                    break;
                case CalcuType.KAIFANG:
                    this .btnShow .Text =Math .Pow ((double ) ValueF ,(double ) ValueL ).ToString() ;
                    break;
                case CalcuType.CHUFANG:
                    this.btnShow.Text = Math.Pow((double)ValueF, 1/(double)ValueL).ToString();
                    break;
            }
            this.ValueF = double.Parse(btnShow.Text);
            this.isNew = true;
            
        }        private void btn归零_Click(object sender, EventArgs e)
        {
            this.ValueF = null;
            this.ValueL = null;
            this.Calculatetype = CalcuType.None;
            this.btnShow.Text  = "0.";
        }
         private void btnJIA_Click(object sender, EventArgs e)
        {
            this. btnResult_Click( sender,  e);
            this.Calculatetype = CalcuType.JIA ;
            this.isNew = true ;
        }        private void btnJIAN_Click(object sender, EventArgs e)
        {
            this. btnResult_Click( sender, e);
            this.Calculatetype = CalcuType.JIAN;
            this.isNew = true;
        }        private void btnCHENG_Click(object sender, EventArgs e)
        {
            this.btnResult_Click( sender,  e);
            this.Calculatetype = CalcuType.CHENG;
            this.isNew = true;
        }        private void btnCHU_Click(object sender, EventArgs e)
        {
            this.  btnResult_Click( sender, e);
            this.Calculatetype = CalcuType.CHU;
            this.isNew = true;
        }        private void btnKAIFANG_Click(object sender, EventArgs e)
        {
            this. btnResult_Click( sender,  e);
            this.Calculatetype = CalcuType.KAIFANG;
            this.isNew = true;
        }        private void btnCHUFANG_Click(object sender, EventArgs e)
        {
            this. btnResult_Click( sender,  e);
            this.Calculatetype = CalcuType.CHUFANG;
            this.isNew = true;
        }        private void btn退格_Click(object sender, EventArgs e)
        {
            if (this.btnShow.Text.Length == 1)
            {
                this.btnShow.Text = "0.";
            }
            else 
            {
                this.btnShow.Text = btnShow.Text.Substring(0, btnShow.Text.Length - 1);
            }
        }         }
}