Form1.cs

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Drawing;
using System.IO;
using System.Security.Cryptography;
using System.Security.Principal;
using System.Windows.Forms;
using FileHasher.Properties;
using Microsoft.Win32;

namespace FileHasher
{
    public partial class Form1 : Form
    {
        public string arg;
        public int HashType;

        public Form1()
        {
            InitializeComponent();
        }

        public Form1(string argument)
        {
            arg = argument;
            InitializeComponent();
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            if (string.IsNullOrEmpty((string) e.Argument))
            {
                e.Result = Resources.InvalidFilenameErrorText;
                return;
            }

            byte[] buffer = {};
            using (var stream = File.OpenRead((string) e.Argument))
            {
                switch (HashType)
                {
                    case 0:
                        buffer = MD5.Create().ComputeHash(stream);
                        break;
                    case 1:
                        buffer = SHA1.Create().ComputeHash(stream);
                        break;
                    case 2:
                        buffer = SHA256.Create().ComputeHash(stream);
                        break;
                }
            }
            e.Result = GetHashStringFromArray(buffer);
        }

        private string GetHashStringFromArray(byte[] data)
        {
            return BitConverter.ToString(data).Replace("-", "");
        }

        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if ((string) e.Result == Resources.InvalidFilenameErrorText)
            {
                progressBar1.Maximum = 100;
                progressBar1.Value = 0;
                progressBar1.Style = ProgressBarStyle.Continuous;
                filenameTextBox.Text = (string) e.Result;
                MessageBox.Show((string) e.Result, "", MessageBoxButtons.OK, MessageBoxIcon.Error);
                groupBox3.Enabled = false;
            }
            else
            {
                progressBar1.Maximum = 100;
                progressBar1.Value = 100;
                progressBar1.Style = ProgressBarStyle.Continuous;
                resultTextBox.Text = (string) e.Result;
                groupBox3.Enabled = true;
            }

            openFileButton.Enabled = true;
            computeButton.Enabled = true;
        }

        private void openFileButton_Click(object sender, EventArgs e)
        {
            var ofdresult = openFileDialog1.ShowDialog();
            if (ofdresult == DialogResult.OK && !string.IsNullOrEmpty(openFileDialog1.FileName))
            {
                filenameTextBox.Text = openFileDialog1.FileName;
            }
        }

        private void computeButton_Click(object sender, EventArgs e)
        {
            openFileButton.Enabled = false;
            computeButton.Enabled = false;
            progressBar1.Style = ProgressBarStyle.Marquee;
            backgroundWorker1.RunWorkerAsync(filenameTextBox.Text);
        }

        private void filenameTextBox_TextChanged(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(filenameTextBox.Text) || filenameTextBox.Text == "" ||
                filenameTextBox.Text == Resources.FilenameTextBoxDefaultText)
            {
                groupBox2.Enabled = false;
                groupBox3.Enabled = false;
            }
            else
            {
                groupBox2.Enabled = true;
                groupBox3.Enabled = false;
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            groupBox1.Text = Resources.Group1Header;
            group1Label.Text = Resources.Group1Label;
            openFileButton.Text = Resources.BrowseButtonDefaultText;
            filenameTextBox.Text = Resources.FilenameTextBoxDefaultText;

            groupBox2.Text = Resources.Group2Header;
            group2Label.Text = Resources.Group2Label;
            computeButton.Text = Resources.ComputeButtonDefaultText;
            progressBar1.Style = ProgressBarStyle.Blocks;

            groupBox3.Text = Resources.Group3Header;
            group3Label.Text = Resources.Group3Label;
            copyButton.Text = Resources.CopyButtonDefaultText;
            resultLabel.Text = Resources.ResultsLable;
            saveButton.Text = Resources.SaveButtonDefaultText;
            compareLabel.Text = Resources.CompareLabel;

            saveFileDialog1.Filter = "Text|*.txt";
            saveFileDialog1.Title = Resources.SaveDialogTitle;

            openFileDialog1.FileName = "";

            enableCMenuCheckBox.Checked = GlobalSettings.WinContextMenuEnabled;

            if (!string.IsNullOrEmpty(arg))
            {
                filenameTextBox.Text = arg;
                computeButton_Click(sender, e);
            }
        }

        private void copyButton_Click(object sender, EventArgs e)
        {
            Clipboard.SetText(resultTextBox.Text);
        }

        private void saveButton_Click(object sender, EventArgs e)
        {
            RETRY:
            var dialogresult = saveFileDialog1.ShowDialog(this);
            if (!string.IsNullOrEmpty(saveFileDialog1.FileName) && dialogresult != DialogResult.Cancel)
            {
                try
                {
                    File.WriteAllText(saveFileDialog1.FileName, resultTextBox.Text);
                }
                catch (Exception ex)
                {
                    if (
                        MessageBox.Show(this, Resources.FileSaveErrorText + "\r\n" + ex.Message,
                            Resources.SaveDialogTitle, MessageBoxButtons.RetryCancel, MessageBoxIcon.Error,
                            MessageBoxDefaultButton.Button2) == DialogResult.Retry)
                        goto RETRY;
                }
            }

            saveFileDialog1.FileName = "";
        }

        private void radioButton1_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton1.Checked)
                HashType = 0;
            if (radioButton2.Checked)
                HashType = 1;
            if (radioButton3.Checked)
                HashType = 2;

            resultTextBox.Text = "";
            groupBox3.Enabled = false;
        }

        private void compareTextBox_TextChanged(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(compareTextBox.Text))
            {
                compareTextBox.BackColor = SystemColors.Control;
                return;
            }
            compareTextBox.BackColor = compareTextBox.Text.ToUpper() == resultTextBox.Text.ToUpper()
                ? Color.LightGreen
                : Color.LightCoral;
        }

        private void enableCMenuCheckBox_CheckedChanged(object sender, EventArgs e)
        {
            if (!IsAdmin())
            {
                RestartElevated(arg);
                return;
            }

            if (enableCMenuCheckBox.Checked)
            {
                RegistryKey regmenu = null;
                RegistryKey regcmd = null;
                try
                {
                    regmenu = Registry.ClassesRoot.CreateSubKey("*\\shell\\Checksum");
                    regmenu?.SetValue("", "Checksum");
                    regcmd = Registry.ClassesRoot.CreateSubKey("*\\shell\\Checksum\\command");
                    regcmd?.SetValue("", $"{Application.ExecutablePath} %1");

                    GlobalSettings.WinContextMenuEnabled = true;
                }
                catch (Exception ex)
                {
                    MessageBox.Show(this, ex.ToString());
                }
                finally
                {
                    regmenu?.Close();
                    regcmd?.Close();
                }
            }
            else
            {
                try
                {
                    var reg = Registry.ClassesRoot.OpenSubKey("*\\shell\\Checksum\\command");
                    if (reg != null)
                    {
                        reg.Close();
                        Registry.ClassesRoot.DeleteSubKey("*\\shell\\Checksum\\command");
                    }
                    reg = Registry.ClassesRoot.OpenSubKey("*\\shell\\Checksum");
                    if (reg != null)
                    {
                        reg.Close();
                        Registry.ClassesRoot.DeleteSubKey("*\\shell\\Checksum");
                    }

                    GlobalSettings.WinContextMenuEnabled = false;
                }
                catch (Exception ex)
                {
                    MessageBox.Show(this, ex.ToString());
                }
            }

            Application.DoEvents();
            enableCMenuCheckBox.Checked = GlobalSettings.WinContextMenuEnabled;
        }

        internal static bool IsAdmin()
        {
            var id = WindowsIdentity.GetCurrent();
            var p = new WindowsPrincipal(id);
            return p.IsInRole(WindowsBuiltInRole.Administrator);
        }

        internal static void RestartElevated(string arg)
        {
            var startInfo = new ProcessStartInfo
            {
                UseShellExecute = true,
                WorkingDirectory = Environment.CurrentDirectory,
                FileName = Application.ExecutablePath,
                Arguments = arg,
                Verb = "runas"
            };
            try
            {
                Process.Start(startInfo);
            }
            catch (Win32Exception)
            {
                return;
            }

            Application.Exit();
        }
    }
}