using System;using System.IO;using System.Diagnostics;

public class Program

{

    public static void Main()

    {

            Stopwatch sw = new Stopwatch(); 

            //string sourceFile = @"D:\CMarkusDownloads\88224835-Ataatapi-Command-Set-2-Acs-2-Rev4.pdf"; //~400 ticks

            string sourceFile = @"D:\CMarkusDownloads\grepWin-2.0.0.msi"; //~400 ticks

​

            byte[] buffer = new byte[4];                                                                    //Declare 4 byte for read buffer

            int bufferLength = 0;                                                                           //Declare total bytes read

            FileStream fr = null; 

​

            sw.Start(); 

            try

            {

                fr = new FileStream(sourceFile, FileMode.Open, FileAccess.Read);                 //Open source file for reading

                bufferLength = fr.Read(buffer, 0, buffer.Length);                                       //Read 4 bytes from source file

                //Console.WriteLine("Read 1st "+bufferLength+" bytes.");                                //slows down timing

                // Check the first 4 bytes, matching '%PDF' string in bytes

                if (bufferLength == 4 && buffer[0] == 0x25 && buffer[1] == 0x50 && buffer[2] == 0x44 && buffer[3] == 0x46)

                {

                    // It's most probably a PDF file

                    sw.Stop();

                    Console.WriteLine(sourceFile + " is a PDF file in " + sw.ElapsedTicks + " ticks.");

                }

                else

                {

                    sw.Stop();

                    Console.WriteLine(sourceFile + " is a not PDF file in " + sw.ElapsedTicks + " ticks.");

                }

            }

            catch (Exception ex)

            {

                sw.Stop();

                Console.WriteLine(ex.Message + " in " + sw.ElapsedTicks+ " ticks.");                                   //Catch exception (if any) and display to user

            }

            finally

            { 

                fr.Dispose(); 

            }

            //Console.ReadKey(); 

    }

}