when i run i get this error the process cannot access the file because it is being used by another process

Question

i send design screen of 2 page as follows in attach file please see the attachments  file.
i send my code below as follows. i run two screen as follows.

In the first screen when i click the payslip button it goes to second page screen.

this is the first screen code as follows

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace Pay_Roll_Applications
{
public partial class Form1 : Form
{
 SqlConnection con = new SqlConnection("Server=(local);initial catalog=Payroll;Trusted_Connection=True");
public Form1()
{
InitializeComponent();
}

 private void cb_name_SelectedIndexChanged(object sender, EventArgs e)

        {

            con.Open();

           SqlCommand cmd1 = new SqlCommand("select * from salaries where Name=' " + cb_name.Text + "'", con);

           SqlDataReader dr = cmd1.ExecuteReader();

            while (dr.Read())

            {

                txt_date.Text = dr[1].ToString().Trim();

                txt_gross.Text = dr[2].ToString().Trim();

                txt_pf.Text = dr[3].ToString().Trim();

                txt_mobilelimit.Text = dr[11].ToString().Trim();

            }

            con.Close();

            con.Open();

            SqlCommand cmd2 = new SqlCommand("select * from salaries where Name=' " + cb_name.Text + "'", con);

            SqlDataReader dr1 = cmd2.ExecuteReader();

            while (dr1.Read())

            {

                txt_date.Text = dr1[1].ToString().Trim();

                txt_gross.Text = dr1[2].ToString().Trim();

                txt_pf.Text = dr1[3].ToString().Trim();

                txt_mobilelimit.Text = dr1[11].ToString().Trim();

            }

            con.Close();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            con.Open();

           SqlCommand cmd = new SqlCommand("insert into salaries values(' " + cb_name.Text + "','" + txt_date.Text + "'," + txt_gross.Text + "," + txt_pf.Text + "," + txt_loan.Text + "," + txt_tax.Text + "," + txt_tds.Text + "," + txt_mobileusage.Text + "," + txt_totdeductions.Text + "," + txt_others.Text + "," + txt_totpayable.Text + "," + txt_mobilelimit.Text + ",'" + txt_usage.Text + "'," + txt_leave.Text + ",'" + txt_remarks.Text + "')", con);

            cmd.ExecuteNonQuery();

            con.Close();

        }

        private void button2_Click(object sender, EventArgs e)

        {

            cb_name.Text = "";

            txt_date.Text = "";

            txt_gross.Text = "";

            txt_pf.Text = "";

            txt_loan.Text = "";

            txt_tax.Text = "";

            txt_tds.Text = "";

            txt_mobileusage.Text = "";

            txt_totdeductions.Text = "";

            txt_others.Text = "";

            txt_totpayable.Text = "";

            txt_mobilelimit.Text = "";

            txt_usage.Text = "";

            txt_leave.Text = "";

            txt_remarks.Text = "";

        }

        private void txt_mobileusage_TextChanged(object sender, EventArgs e)

        {

            int a, b, c, j, tt, z;

            if (!int.TryParse(txt_pf.Text, out a) || a < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_pf.Focus();

                return;

            }

            if (!int.TryParse(txt_loan.Text, out b) || b < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_loan.Focus();

                return;

            }

            if (!int.TryParse(txt_tax.Text, out c) || c < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_tax.Focus();

                return;

            }

            if (!int.TryParse(txt_tds.Text, out j) || j < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_tds.Focus();

                return;

            }

            if (!int.TryParse(txt_mobileusage.Text, out tt) || tt < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter and Enter only Numbers.");

                txt_mobileusage.Focus();

                return;

            }

            z = a + b + c + j + tt;

            txt_totdeductions.Text = z.ToString();

        }

        private void txt_others_TextChanged(object sender, EventArgs e)

        {

            int a, b, c, d;

            if (!int.TryParse(txt_gross.Text, out a) || a < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_gross.Focus();

                return;

            }

            if (!int.TryParse(txt_totdeductions.Text, out b) || b < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter.");

                txt_totdeductions.Focus();

                return;

            }

            if (!int.TryParse(txt_others.Text, out c) || c < 0)

            {

                MessageBox.Show("Not a valid number. Please reenter and Enter only Numbers.");

                txt_others.Focus();

                return;

            }

            d = a - b + c;

            txt_totpayable.Text = d.ToString();

        }

        private void button3_Click(object sender, EventArgs e)

        {

            Form2 f2 = new Form2();

            f2.Show();

            this.Hide();

        }

    }

}

this is the Second screen code as follows 

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Text;

using System.Windows.Forms;

using System.Data.SqlClient;

namespace Pay_Roll_Applications

{

    public partial class Form2 : Form

    {

        SqlConnection con = new SqlConnection("Server=(local);initial catalog=Payroll;Trusted_Connection=True");

        public Form2()

        {

            InitializeComponent();

        }

        private void Form2_Load(object sender, EventArgs e)

        {

            int i;

            for (i = 1; i <= 31; i++)

            {

                comboBox1.Items.Add(i);

            }

            txt_basicscale.Text = "0";

            txt_hrascale.Text = "0";

            txt_dascale.Text = "0";

            txt_variablescale.Text = "0";

            txt_working.Text = "31";

            txt_basicamount.Text = "0";

            txt_hraamt.Text = "0";

            txt_daamt.Text = "0";

            txt_variableamt.Text = "0";

        }

        private void button3_Click(object sender, EventArgs e)

        {

            con.Open();

            SqlCommand cmd = new SqlCommand("insert into pay values('" + cb_id.Text + "'," + txt_working.Text + ",' " + txt_Name.Text + "','" + txt_fathers.Text + "','" + txt_pan.Text + "','" + txt_date.Text + "'," + txt_days.Text + ",'" + txt_branch.Text + "',' " + txt_designation.Text + "','" + txt_department.Text + "',' " + txt_pay.Text + "','" + txt_bankname.Text + "'," + txt_accno.Text + "," + txt_basicamount.Text + "," + txt_hraamt.Text + "," + txt_daamt.Text + ", " + txt_variableamt.Text + "," + txt_percentagebasic.Text + "," + txt_excessmobile.Text + "," + txt_tds.Text + "," + txt_loan.Text + "," + txt_netpay.Text + ")", con);

            cmd.ExecuteNonQuery();

        }

        private void cb_id_SelectedIndexChanged(object sender, EventArgs e)

        {

            con.Open();

            SqlCommand cmd1 = new SqlCommand("select * from pay where Employeeid = '" + cb_id.Text + "'", con);

            SqlDataReader dr = cmd1.ExecuteReader();

            while (dr.Read())

            {

                txt_Name.Text = dr[2].ToString().Trim();

                txt_fathers.Text = dr[3].ToString().Trim();

                txt_pan.Text = dr[4].ToString().Trim();

                txt_date.Text = dr[5].ToString().Trim();

                txt_branch.Text = dr[7].ToString().Trim();

                txt_designation.Text = dr[8].ToString().Trim();

                txt_department.Text = dr[9].ToString().Trim();

                txt_pay.Text = dr[10].ToString().Trim();

                txt_bankname.Text = dr[11].ToString().Trim();

                txt_accno.Text = dr[12].ToString().Trim();

                txt_basicscale.Text = dr[13].ToString().Trim();

                txt_hrascale.Text = dr[14].ToString().Trim();

                txt_dascale.Text = dr[15].ToString().Trim();

                txt_variablescale.Text = dr[16].ToString().Trim();

                txt_excessmobile.Text = dr[18].ToString().Trim();

                txt_tds.Text = dr[19].ToString().Trim();

                txt_loan.Text = dr[20].ToString().Trim();

                txt_basicamount.Text = "";

                txt_hraamt.Text = "";

                txt_daamt.Text = "";

                txt_variableamt.Text = "";

                txt_scaletot.Text = "";

                txt_amttot.Text = "";

                textBox1.Text = "";

                txt_netpay.Text = "";

                txt_days.Text = "";

                txt_percentagebasic.Text = "";

            }

            con.Close();

            con.Open();

            SqlCommand cmd2 = new SqlCommand("select * from pay where Employeeid='" + cb_id.Text + "'", con);

            SqlDataReader dr1 = cmd2.ExecuteReader();

            while (dr1.Read())

            {

                txt_percentagebasic.Text = dr1[17].ToString().Trim();

            }

            con.Close();

            

        }

        private void button5_Click(object sender, EventArgs e)

        {

            Form1 f1 = new Form1();

            f1.Show();

            this.Hide();

        }

        private void dateTimePicker1_ValueChanged(object sender, EventArgs e)

        {

            DateTime dt = dateTimePicker1.Value;

            int days = DateTime.DaysInMonth(dt.Year, dt.Month);

            txt_working.Text = days.ToString();

        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)

        {

            txt_days.Text = comboBox1.Text;

            int r, d1;

            r = Convert.ToInt32(txt_variablescale.Text);

            d1 = Convert.ToInt32(1 * r);

            txt_variableamt.Text = d1.ToString();

            int c, d;

            c = Convert.ToInt32(txt_basicscale.Text);

            d = Convert.ToInt32(0.12 * c);

            txt_percentagebasic.Text = d.ToString();

            double working_days = Convert.ToDouble(txt_working.Text);

            double payable_days = Convert.ToDouble(txt_days.Text);

            double a = Convert.ToDouble(txt_basicscale.Text);

            txt_basicamount.Text = Math.Floor(a / working_days * payable_days).ToString();

            double working_days1 = Convert.ToDouble(txt_working.Text);

            double payable_days1 = Convert.ToDouble(txt_days.Text);

            double a1 = Convert.ToDouble(txt_hrascale.Text);

            txt_hraamt.Text = Math.Floor(a1 / working_days1 * payable_days1).ToString();

            double working_days2 = Convert.ToDouble(txt_working.Text);

            double payable_days2 = Convert.ToDouble(txt_days.Text);

            double a2 = Convert.ToDouble(txt_dascale.Text);

            txt_daamt.Text = Math.Floor(a2 / working_days2 * payable_days2).ToString();

            double working_days3 = Convert.ToDouble(txt_working.Text);

            double payable_days3 = Convert.ToDouble(txt_days.Text);

            double a3 = Convert.ToDouble(txt_variablescale.Text);

            txt_variableamt.Text = Math.Floor(a3 / working_days3 * payable_days3).ToString();

            txt_amttot.Text = "";

            textBox1.Text = "";

            txt_netpay.Text = "";

        }

        private void button1_Click(object sender, EventArgs e)

        {

            double BasicScale, HraScale, DaScale, VpiScale, BasicAmt, Hraamt, daAmt, VpiAmt, PercentBasic, Excess, Tds, Loan, total, total1, total2, total3, total4;

            BasicScale = Convert.ToDouble(txt_basicscale.Text);

            HraScale = Convert.ToDouble(txt_hrascale.Text);

            DaScale = Convert.ToDouble(txt_dascale.Text);

            VpiScale = Convert.ToDouble(txt_variablescale.Text);

            BasicAmt = Convert.ToDouble(txt_basicamount.Text);

            Hraamt = Convert.ToDouble(txt_hraamt.Text);

            daAmt = Convert.ToDouble(txt_daamt.Text);

            VpiAmt = Convert.ToDouble(txt_variableamt.Text);

            PercentBasic = Convert.ToDouble(txt_percentagebasic.Text);

            Excess = Convert.ToDouble(txt_excessmobile.Text);

            Tds = Convert.ToDouble(txt_tds.Text);

            Loan = Convert.ToDouble(txt_loan.Text);

            total = BasicScale + HraScale + DaScale + VpiScale;

            txt_scaletot.Text = total.ToString();

            total1 = BasicAmt + Hraamt + daAmt + VpiAmt;

            txt_amttot.Text = total1.ToString();

            total2 = PercentBasic + Excess + Tds + Loan;

            textBox1.Text = total2.ToString();

            int a, b, c;

            if (!int.TryParse(txt_amttot.Text, out a) || a < 0)

            {

                txt_amttot.Focus();

                return;

            }

            if (!int.TryParse(textBox1.Text, out b) || b < 0)

            {

                textBox1.Focus();

                return;

            }

            c = a - b;

            txt_netpay.Text = c.ToString();

        }

    }

}