A simple file MD5 code automatic calculation comparator (with source code)

The main update of winform is not very clear about multi-threading, winding around, get very confused, the main code is as follows, also ask each hero to give directions, thank you!

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;
 using System.Security.Cryptography;
 using System.IO;
 using System.Threading;

 namespace FileMD5 {
     public partial class MainForm : Form {
         public MainForm() {
             InitializeComponent();
         }

         private void button_file_Click(object sender, EventArgs e) {
             OpenFileDialog fileDialog = new OpenFileDialog();
             fileDialog.Title = " Please select file ";
             fileDialog.RestoreDirectory = true;

             if (fileDialog.ShowDialog() == DialogResult.OK) {
                 textBox_file.Text = fileDialog.FileName;
                 textBox_result.Text = "";
                 FileInfo file = new FileInfo(fileDialog.FileName);
                 fileSzie = file.Length;
                 showFilesize(fileSzie);
             }

         }

         private void button_exit_Click(object sender, EventArgs e) {
             this.Close();
             this.Dispose();
         }

         private void button_check_Click(object sender, EventArgs e) {
             checkResult();
         }

         MD5 md5 = (MD5)CryptoConfig.CreateFromName("MD5");
         long fileSzie = 0;

         private void button_calc_Click(object sender, EventArgs e) {
             string file = textBox_file.Text;

             if (file.Length == 0) {
                 textBox_result.Text = " Please reselect the file first !";
                 return;
             }

             FileStream fs = null;
             try {
                 fs = new FileStream(file, FileMode.Open, FileAccess.Read);
             } catch (SystemException) {
                 textBox_result.Text = " File opening error , Please reselect the file !";
                 return;
             }

             // For more than  100M  File to enable multithreading 
             if (fs.Length > 100L * 1024 * 1024) {

                 string message = " File has exceeded  100M  , which requires a long calculation time. \n The software will start the background thread for processing. Do you want to continue? ";
                 string caption = " The file is bigger ";
                 MessageBoxButtons buttons = MessageBoxButtons.YesNo;

                 if (MessageBox.Show(message, caption, buttons) == System.Windows.Forms.DialogResult.No) {
                     fs.Close();
                     textBox_result.Text = " Large file, not counted. ";
                     return;
                 }
                 textBox_result.Text = " Please wait a moment while we calculate ......";
                 button_calc.Enabled = false;
                 button_file.Enabled = false;

                 Thread thread = new Thread(new ParameterizedThreadStart(calcMD5));
                 thread.Start(fs);

             } else {
                 calcMD5(fs);
             }
         }

         // To establish 1 a  object  Parameter functions are designed to handle the problem of using parameters in thread calls. 
         private void calcMD5(object fs) {
             calcMD5((FileStream)fs);
         }

         // Invoke  The delegate that the function needs to use 
         delegate void updateWindows(byte[] result);

         private void calcMD5(FileStream fs) {
             byte[] md5byte = md5.ComputeHash(fs);

             if (this.InvokeRequired) {
                 this.Invoke(new updateWindows(showResult), md5byte);
             } else {
                 showResult(md5byte);
             }
             fs.Close();
         }

         private void showResult(byte[] md5byte) {
             int i, j;
             StringBuilder sb = new StringBuilder(32);
             foreach (byte b in md5byte) {
                 i = Convert.ToInt32(b);
                 j = i >> 4;
                 sb.Append(Convert.ToString(j, 16));
                 j = ((i << 4) & 0x00ff) >> 4;
                 sb.Append(Convert.ToString(j, 16));
             }

             String result = sb.ToString().ToUpper();

             textBox_result.Text = result;
             button_calc.Enabled = true;
             button_file.Enabled = true;
             checkResult();

         }

         private void checkResult() {

             string result = textBox_result.Text;

             if (textBox_md5.Text.Length == 0) {
                 textBox_compare.Text = "";
                 textBox_compare.Visible = false;
                 return;
             }

             if(result.Length != 32 ) {
                 textBox_compare.Visible = true;
                 textBox_compare.BackColor = Color.Pink;
                 textBox_compare.Text = " Not in the result box MD5 Code, please calculate first! ";
                 return;
             }

             if (textBox_md5.Text.Trim().ToUpper().Equals(result.ToUpper())) {
                 textBox_compare.Visible = true;
                 textBox_compare.BackColor = Color.LightGreen;
                 textBox_compare.Text = "MD5 code   Has been matched, the file has not been modified, can be assured to use! ";
             } else {
                 textBox_compare.Visible = true;
                 textBox_compare.BackColor = Color.Red;
                 textBox_compare.Text = "MD5 code   Does not match, the file has been modified, please be careful! ";
             }
         }

         private void showFilesize(long size) {

             float d_size;
             string unit = "Byte";

             if (size > 1024 * 1024 * 1024) {    // Is greater than  1G  The display of the 
                 d_size = size / (float)(1024 * 1024 * 1024);
                 unit = "GB";
             } else {
                 if (size > 1024 * 1024) {    // Is greater than  1M  The display of the 
                     d_size = size / (float)(1024 * 1024);
                     unit = "MB";
                 } else {
                     if (size > 1024) {    // Is greater than  1K  The display of the 
                         d_size = size / (float)(1024);
                         unit = "KB";
                     } else {
                         d_size = size;
                     }
                 }
             }
             textBox_filesize.Text = string.Format(" {0:F} {1} ( {2:N0} byte  )", d_size, unit, size);
         }
     }
 }

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