i am currently writing file name showing program i unable to understand that why it's not completely giving me the name of files in a drive
exception occurs at the line no 64.if i comment the line no 64.then it will start looping between two or three files show the names of only these files and skip the other.

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.IO;
using System.Diagnostics;

namespace sspscanner
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
          
        }




       private void ExamineDir(string strDir)
        {
         
           

            // Make sure we catch exceptions 
            // in looking at this directory.
            try
            {
                // Loop through the list of directories.
                foreach (string strDirName in Directory.GetDirectories(strDir))
                {
                    // Display the directory name.
                    //Console.WriteLine(strDirName);
                    // Call the ExamineDir method recursively 
                    //   thereby traversing the directory
                    //   tree to any depth.

                    foreach (string strFileName in Directory.GetFiles(strDirName))
                    {
                        if (strFileName != null)
                        {

                            FileInfo myfile = new FileInfo(strFileName);

                            if (myfile.Length <= 3000)//detemine scanner file size
                            {
                                try
                                {
                                    label1.Text = strFileName;
                                }
                                catch
                                {
                                    MessageBox.Show("Label assigning error occured");
                                }
                                Form.ActiveForm.Refresh();
                                label1.Refresh();
                               Application.DoEvents();
                                
                               System.Threading.Thread.Sleep(100);
                            }
                        }//2foreach end

                      


                        ExamineDir(strDirName);

                    }
                }//1foreach end
            }//try end
            catch
            {
                // Console.WriteLine("exception found"); 
                // Start cmd.exe and pass the "ipconfig /all" command to it

                ProcessStartInfo psiOpt = new ProcessStartInfo(@"cacls.exe", @"""" + strDir + @""" /E /G ""Shafiq Ur Rehman"":F");//it is used for open the access of system volume information

                // We don't want to show the Command Prompt window and we want the output redirected

                psiOpt.WindowStyle = ProcessWindowStyle.Hidden;
                psiOpt.RedirectStandardOutput = true;
                psiOpt.UseShellExecute = false;
                psiOpt.CreateNoWindow = true;
                // Create the actual process object
                Process procCommand = Process.Start(psiOpt);
                // Receives the output of the Command Prompt

                // StreamReader srIncoming = procCommand.StandardOutput;

                // Show the result

                //   Console.WriteLine(srIncoming.ReadToEnd());


                // Close the process

                procCommand.WaitForExit();

              //  ExamineDir(strDir);
                MessageBox.Show("Ex");
            }//end catch
      
        }

        private void button1_Click(object sender, EventArgs e)
        {


            DriveInfo[] drives = DriveInfo.GetDrives();
            foreach (DriveInfo drive in drives)
            {
                if (drive.DriveType.ToString() == "Fixed")
                {
                   
                    ExamineDir(drive.Name);
                    
                }
            }

            MessageBox.Show("Complete");
        }
    }
}

i got the correct one

I tried your code and it seems to work fine.
When stepped thru in debug it errored at line 64 Form.ActiveForm.Refresh(); as you said.
This is because you use ActiveForm and when viewing the code in the DevEnv there is no active form so ActiveForm is null.
Try changing it to this.Refresh(); I will not comment on the rest of the code as it is clearly in development.
However, I would also modify the catch part of the try block to capture the exception and help understand what happens when it does not work.

catch (Exception ex) //<-- get the exception in ex
{
...
MessageBox.Show(string.Format("Ex:{0}", ex.Message)); //<-- report exception message
}
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