Vfrdtyky

I am new to c# programming.I am building a simple calculator but when i add two numbers and press operator(+) button it adds two numbers but when i again press another operator it just adds the previous value like 2+2= 4 and if i press (-) operator after that it just shows result = 8 , please help me to get out of this.
thanks.

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 Calculator

    {

        public partial class Form1 : Form

        {

          double value  = 0;

          string operation = "";

          bool opt_pressed = false;







            public Form1()

            {

                InitializeComponent();

            }



            private void button1_Click(object sender, EventArgs e)

            {

                if ((txtMain.Text == "0") || (opt_pressed))

                    txtMain.Clear();

                Button a = (Button)sender;

                opt_pressed = false;

                if (a.Text == ".")

                {

                    if (!txtMain.Text.Contains("."))

                        txtMain.Text = txtMain.Text + a.Text;





                }





                else

                {

                    txtMain.Text = txtMain.Text + a.Text;



                }



            }



            private void txtMain_TextChanged(object sender, EventArgs e)

            {



            }



            private void button10_Click(object sender, EventArgs e)

            {   

                txtMain.Clear();

                value = 0;

                label1.Text = "";











            }



            private void button11_Click(object sender, EventArgs e)

            {   

                Button b = (Button)sender;



                if (value != 0)

             {

                equal.PerformClick();

                operation = b.Text;



                label1.Text = value + " " + operation;

                opt_pressed = true;



             }

                else 



        {       operation = b.Text;

                value = double.Parse(txtMain.Text);



                label1.Text = value + " " + operation;

                opt_pressed = true;

        }



            }



            private void button16_Click(object sender, EventArgs e)

            {



                switch (operation) 

                {

                    case  "+":



                        txtMain.Text = (value + double.Parse(txtMain.Text)).ToString();

                        break;

                    case "-":



                        txtMain.Text = (value - double.Parse(txtMain.Text)).ToString();

                        break;

                    case "/":



                        txtMain.Text = (value / double.Parse(txtMain.Text)).ToString();

                        break;

                    case "*":



                        txtMain.Text = (value * double.Parse(txtMain.Text)).ToString();

                        break;



               }



                value = double.Parse(txtMain.Text);

                label1.Text = "";





            }



            private void label1_Click(object sender, EventArgs e)

            {



            }



            private void Form1_Load(object sender, EventArgs e)

            {



            }



            private void button1_Click(object sender, KeyPressEventArgs e)

            {

                Button a = (Button)sender;



                button1.PerformClick();

                operation = a.Text;



            }









        }

    }