FileStream fs = new FileStream(Server.MapPath("example.txt"), FileMode.Open);
 //By using FileStream, we start to read the file. "FileMode.Open" means that this
 //file will be used just for reading not another operations. Other possible values are:
 //Append,Create,Truncate,CreateNew,OpenOrCreate 

byte[] data = new byte[fs.Length]; fs.Read(data, 0, (int)fs.Length);
//we read the whole file by "from 0 to fs.length" 

foreach (byte k in data) 
{ 
    Response.Write(k.ToString()); 
} 
//and we send the result to the user 

StreamWriter writer = new StreamWriter(File.Open(Server.MapPath("enes.txt"), FileMode.Open)); 
//file.open() -->> creates a filestream for streamwriter 
writer.Write("write to the file");
writer.Close(); 

////////////////////////////////////////////////////////// 

StreamReader reader = new StreamReader(File.Open(Server.MapPath("enes.txt"), FileMode.Open));
Label1.Text = reader.ReadLine(); 
reader.Close(); 

 string k = File.ReadAllText(Server.MapPath("enes.txt")); 

and

 File.WriteAllText(@"C:\temp.txt",data); 

• DriveInfo
• DirectoryInfo
• FileInfo
• Directory
• File

To create a directory or file
Directory and File classes in System.IO namespace allow us to create and delete directories or files:

Directory.Create(@"C:\exampleDirectory");

Directory.Delete(@"C:\exampleDirectory");

If we want to work on files and directories in current project folder, 

Directory.Create(@"Server.MapPath("exampleDirectory"));  

Directory.Delete(@"Server.MapPath("exampleDirectory"));  

we can use. When operating on files, use File class:

File.Create(@"C:\exampleFile.doc")

File.Delete(@"C:\exampleFile.doc")

or

File.Create(@"Server.MapPath("exampleFile"));  

File.Delete(@"Server.MapPath("exampleFile"));  

Now let's work on displaying directory and file hierarchy in treeView like "windows explorer" style. We will use these classes:

• System.IO.DriveInfo        //allows us to access the name, available space, total space etc. of a disk volume
• System.IO.DirectoryInfo    //allows us to access the name, available space, total space etc. of directory
  

Create a new site, add TreeView and wrote below code in Page_Load event:

rotected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            System.IO.DriveInfo drive = new System.IO.DriveInfo(@"D:\"); 
            //work on local disk D
            TreeNode node = new TreeNode(); 
            //we create TreeNode's dinamically

            node.Value = drive.Name;  
            TreeView1.Nodes.Add(node); 
            //add local disk D to main node

            loadDirectories (node, drive.Name);//in this recursive method,
            // we pass name and full address of the local disk 
            // as parameters            
        }
    }

    private void loadDirectories(TreeNode parent, string path)
    {
        System.IO.DirectoryInfo directory = new System.IO.DirectoryInfo(path); 
        //now we don't work on drives but directories

        try
        {
           //with  GetDirectories(), we get all sub directories
            foreach (System.IO.DirectoryInfo d in directory.GetDirectories()) 
            {
                TreeNode node = new TreeNode(d.Name, d.FullName);                

                parent.ChildNodes.Add(node);  

                //call the same method (call "loadDirectories()" recursively )
                loadDirectories(node, d.FullName);        
            }
        }
        catch (System.UnauthorizedAccessException e)
        {
            parent.Text += "(acces forbiddeni)";
        }

        catch (System.IO.IOException e)
        {
            parent.Text += "(unknown error hata: " + e.Message + ")";
        }   
    }

We displayed just content of volume D by writing ?System.IO.DriveInfo drive = new System.IO.DriveInfo(@"D:\");". If we wanted to display all files and directories in computer hard disk, we would write:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            foreach (System.IO.DriveInfo drive in System.IO.DriveInfo.GetDrives())  
            //to work on all volumes
            {
                TreeNode node = new TreeNode();
                node.Value = drive.Name;

                if (drive.IsReady)
                {
                    node.Text = drive.Name;
                    loadDirectories(node, drive.Name);
                }                            

                TreeView1.Nodes.Add(node);
            } 
        }
}

protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)
    {
        DirectoryInfo directory = new DirectoryInfo(TreeView1.SelectedNode.Value);

        GridView1.DataSource = directory.GetFiles();
        GridView1.DataBind();
    }