source: Common/ExtTools.cs

using System;
using System.IO;
using System.Diagnostics;
using System.Windows.Forms;
using Microsoft.Win32;
using System.Runtime.InteropServices;

namespace Common
{
    static class ExtTools
    {
        public static string CompareProgram;
        public static string CompareAttributes;

        static ExtTools()
        {
            // Autodetect location of Beyond Compare executable
            CompareProgram = "C:\\Program Files\\Beyond Compare 4\\BCompare.exe";
            if (!File.Exists(CompareProgram)) CompareProgram = "C:\\Program Files (x86)\\Beyond Compare 4\\BCompare.exe";
            if (!File.Exists(CompareProgram)) CompareProgram = "C:\\Program Files\\Beyond Compare 3\\BCompare.exe";
            if (!File.Exists(CompareProgram)) CompareProgram = "C:\\Program Files (x86)\\Beyond Compare 3\\BCompare.exe";

            CompareAttributes = "\"<<left_file>>\" \"<<right_file>>\"";
        }

        public static string GetTempDir()
        {
            return Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) + "\\" + 
                Application.ProductName + "\\Temp";
        }

        public static void CompareText(string textLeft, string textRight, string fileNameLeft = "FileLeft.txt", 
            string fileNameRight = "FileRight.txt")
        {
            if (!Directory.Exists(GetTempDir()))
            {
                Directory.CreateDirectory(GetTempDir());
            }
            string tempFileLeft = GetTempDir() + "\\" + fileNameLeft;
            string tempFileRight = GetTempDir() + "\\" + fileNameRight;
            FileExt.WriteAllTextUpdate(tempFileLeft, textLeft);
            FileExt.WriteAllTextUpdate(tempFileRight, textRight);
            CompareFiles(tempFileLeft, tempFileRight);
        }

        public static void CompareFiles(string filenameLeft, string filenameRight)
        {
            try
            {
                // Execute comparison process
                string attributes = CompareAttributes;
                attributes = attributes.Replace("<<left_file>>", filenameLeft);
                attributes = attributes.Replace("<<right_file>>", filenameRight);
                ExecuteProgram(CompareProgram, attributes);
            } catch (Exception e)
            {
                MessageBox.Show(e.Message);
            }
        }

        public static void ExecuteProgram(string executableFileName, string arguments = "", string workingDirectory = "")
        {
            // Execute external process
            Process process = new Process();
            process.StartInfo.FileName = executableFileName;
            process.StartInfo.Arguments = arguments;

            if (!File.Exists(process.StartInfo.FileName))
                throw new Exception("Executable file \"" + process.StartInfo.FileName + "\" not found.");

            process.EnableRaisingEvents = true;
            process.StartInfo.WorkingDirectory = workingDirectory;
            process.StartInfo.CreateNoWindow = false;
            process.StartInfo.UseShellExecute = true;
            process.StartInfo.WindowStyle = ProcessWindowStyle.Maximized;
            process.Start();
        }

        public static string ExecuteProgramWait(string executableFileName, string arguments, string dir = "")
        {
            string output = "";
            // Execute external process
            Process process = new Process();
            process.StartInfo.FileName = executableFileName;
            process.StartInfo.Arguments = arguments;
            process.StartInfo.UseShellExecute = false;
            process.StartInfo.RedirectStandardOutput = true;
            process.StartInfo.RedirectStandardError = true;

            try
            {
                process.EnableRaisingEvents = true;
                process.StartInfo.WorkingDirectory = dir;
                process.StartInfo.CreateNoWindow = true;
                process.StartInfo.WindowStyle = ProcessWindowStyle.Maximized;
                process.Start();
                output = process.StandardOutput.ReadToEnd();
                output += process.StandardError.ReadToEnd();
                process.WaitForExit();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            return output;
        }

        public static void SetFileAssociation(string extension, string progId, string openWith, string fileDescription)
        {
            RegistryKey BaseKey;
            RegistryKey OpenMethod;
            RegistryKey Shell;
            RegistryKey CurrentUser;

            BaseKey = Registry.ClassesRoot.CreateSubKey(extension);
            BaseKey?.SetValue("", progId);

            OpenMethod = Registry.ClassesRoot.CreateSubKey(progId);
            OpenMethod?.SetValue("", fileDescription);
            OpenMethod?.CreateSubKey("DefaultIcon")?.SetValue("", "\"" + openWith + "\",0");
            Shell = OpenMethod?.CreateSubKey("Shell");
            Shell?.CreateSubKey("edit")?.CreateSubKey("command")?.SetValue("", "\"" + openWith + "\"" + " \"%1\"");
            Shell?.CreateSubKey("open")?.CreateSubKey("command")?.SetValue("", "\"" + openWith + "\"" + " \"%1\"");
            Shell?.Close();
            OpenMethod?.Close();
            BaseKey?.Close();

            // Delete explorer override
            CurrentUser = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\FileExts\\" + extension, true);
            CurrentUser?.DeleteSubKey("UserChoice", false);
            CurrentUser?.Close();

            // Tell explorer the file association has been changed
            SHChangeNotify(0x08000000, 0x0000, IntPtr.Zero, IntPtr.Zero);
        }

        [DllImport("shell32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        public static extern void SHChangeNotify(uint wEventId, uint uFlags, IntPtr dwItem1, IntPtr dwItem2);
    }
}