在文本框中只允许一个小数点

我正在用c#设计一个基本的计算器。当我进入小数点时，我遇到了困难。例如，如果我输入。8，那么它给我0.8，这是正确的，如果没有显示屏幕上，但在那之后，如果我输入十进制符号，那么它也接受它，这是0.8.....我希望一个数字只接受一个十进制符号。我的代码是

private void btn_Decimal_Click(object sender, EventArgs e)
{
    if (txt_Result.Text == "" || LastcharIssymbol==true)
    {
         txt_Result.Text = txt_Result.Text + 0 + ".";
    }
    else
         txt_Result.Text = txt_Result.Text + ".";
}

在这里，如果我输入0.9.999，那么它也接受，如果我输入999.....然后它也接受了。我希望只接受一个十进制符号来表示999.999这个数字。请帮帮我。此外，我还添加了两个额外的标签，可以显示当前的系统日期和时间。我无法显示日期以及时间，但我能够使用VB.Net显示日期和时间。我不知道我从哪里得到错误。我的全部代码是

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 CS_Calculator
{
    public partial class Form1 : Form
    {
        Boolean LastcharIssymbol {get;set;}
        string op;
        double a, memory;
        public Form1()
        {
            InitializeComponent();
        }
        private void btn_1_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "1";
            LastcharIssymbol= false;
        }
        private void btn_2_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "2";
            LastcharIssymbol = false;
        }
        private void btn_3_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "3";
            LastcharIssymbol = false;
        }
        private void btn_4_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "4";
            LastcharIssymbol = false;
        }
        private void btn_5_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "5";
            LastcharIssymbol = false;
        }
        private void btn_6_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "6";
            LastcharIssymbol = false;
        }
        private void btn_7_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "7";
            LastcharIssymbol = false;
        }
        private void btn_8_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "8";
            LastcharIssymbol = false;
        }
        private void btn_9_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "9";
            LastcharIssymbol = false;
        }
        private void btn_0_Click(object sender, EventArgs e)
        {
            txt_Result.Text = txt_Result.Text + "0";
            LastcharIssymbol = false;
        }
        private void btn_Decimal_Click(object sender, EventArgs e)
        {
            if (txt_Result.Text == "" || LastcharIssymbol==true)
            {
                txt_Result.Text = txt_Result.Text + 0 + ".";
            }
            else
                txt_Result.Text = txt_Result.Text + ".";
        }
        private void btn_Plus_Click(object sender, EventArgs e)
        {
            if(txt_Result.Text=="" || LastcharIssymbol)
            {
                MessageBox.Show("Please Enter first number to perform the addition operation.");
            }
            else
            {
                op = "+";
                txt_Result.Text = txt_Result.Text + op;
                LastcharIssymbol=true;
            }
        }
        private void btn_Minus_Click(object sender, EventArgs e)
        {
            if (txt_Result.Text == "" || LastcharIssymbol)
            {
                MessageBox.Show("Please enter first number to erform the substraction operation.");
            }
            else
            {
                op = "-";
                txt_Result.Text = txt_Result.Text + op;
                LastcharIssymbol = true;
            }
        }
        private void btn_Division_Click(object sender, EventArgs e)
        {
            if (txt_Result.Text == "" || LastcharIssymbol)
            {
                MessageBox.Show("Please enter first number to perform the division operation.");
            }
            else
            {
                op = "/";
                txt_Result.Text = txt_Result.Text + op;
                LastcharIssymbol = true;
            }
        }
        private void btn_Mult_Click(object sender, EventArgs e)
        {
            if (txt_Result.Text == "" || LastcharIssymbol)
            {
                MessageBox.Show("Please enter first number to perform the multiplication operation.");
            }
            else
            {
                op = "*";
                txt_Result.Text = txt_Result.Text + op;
                LastcharIssymbol = true;
            }
        }
        private void btn_Equal_Click(object sender, EventArgs e)
        {
        }
        private void button4_Click(object sender, EventArgs e)
        {
            txt_Result.Text = "";
        }
        private void btn_Clear_All_Click(object sender, EventArgs e)
        {
            txt_Result.Text = "";
            op = "";
            a = 0;
            memory = 0;
        }
        private void btn_Memory_Click(object sender, EventArgs e)
        {
            memory = Convert.ToDouble(txt_Result.Text);
        }
        private void btn_Show_Memory_Click(object sender, EventArgs e)
        {
            txt_Result.Text = memory.ToString();
        }
    }
}