How to attach/delete/upload files to SharePoint List Item using Object Model

I have one requirement in which I need to develop a web part in which users can upload multiple documents to SharePoint List Item and they can see all uploaded files into data grid control, they also need facility for user to delete selected files, add new filesfrom same control and also they can download files.

Here in this post, I am writing code for each functionality as per my requirements.

Snippet code of deleting attachment from SharePoint list item : 

private void DeleteAttachment(int NodeID, String strFileName)
    {
        try
        {
            SPContext.Current.Web.AllowUnsafeUpdates = true;

            SPListItem delItem = lstAudit.GetItemById(NodeID);
            SPAttachmentCollection atCol = delItem.Attachments;
            foreach (string strDelfileName in atCol)
            {
                if (strDelfileName == strFileName)
                {
                    atCol.Delete(strDelfileName);
                    delItem.Update();
                    lstAudit.Update();

                    return;
                }
            }

        }
        catch (Exception eDel)
        {
            string errDel = eDel.Message;
        }
    }
  

Snippet code for downloading attachment from SharePoint List Item :
 private void DownloadAttachment(string FileName)
    {
        try
        {
            string AttachmentURL = string.Empty;
            AttachmentURL = FileName;
            string strName = AttachmentURL.Substring(AttachmentURL.LastIndexOf("/") + 1);
            string sbURL = AttachmentURL.Trim();
            System.Web.HttpContext.Current.Response.ContentEncoding = System.Text.Encoding.Default;
            Response.AppendHeader("Content-disposition", "attachment; filename=" + strName);
            Response.AppendHeader("Pragma", "cache");
            Response.AppendHeader("Cache-control", "private");
            Response.WriteFile(sbURL);
            Response.End();
        }
        catch (Exception eDwn)
        {
            Response.Write(eDwn.Message);
        }
    }

Snippet code of uploading document to SharePoint List Item :
 protected void btnUpload_Click(object sender, EventArgs e)
    {
        try
        {
            string fileName = "";
            string strExtensionName = "";

            fileName = System.IO.Path.GetFileName(FileUpload.PostedFile.FileName);

            if (fileName != "")
            {
                strExtensionName = fileName.Substring(fileName.IndexOf(".") + 1);
                if (strExtensionName.Equals("webpart") || strExtensionName.Equals("dll") || strExtensionName.Equals("bat") || strExtensionName.Equals("exe"))
                {
                    lblMessage.Visible = true;
                    lblMessage.Text = "Invalid fileName!!";
                }
                else
                {
                    string _fileTime = DateTime.Now.ToFileTime().ToString();

                    string _fileorgPath = System.IO.Path.GetFullPath(FileUpload.PostedFile.FileName);

                    if (txtFileName.Text.Trim().Length > 0)
                    {
                        fileName = fileName.Replace(fileName.Substring(fileName.LastIndexOf("\\") + 1), txtFileName.Text) + "." + strExtensionName;
                    }
                    string _newfilePath = _fileTime + "~" + fileName;

                    string tempFolder = Environment.GetEnvironmentVariable("TEMP");

                  
                    string _filepath = tempFolder + _newfilePath;
                  
                    FileUpload.PostedFile.SaveAs(_filepath);

                    AddRow(fileName, _filepath, 0, true);
                }
            }
            else
            {

                lblMessage.Visible = true;
                lblMessage.Text = "Please Selct file Name";

            }

        }
        catch (Exception Ex)
        {
            Response.Write(Ex.Message);
        }

    }

  
    protected void dgdUpload_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        int recordToDelete = e.RowIndex;

        //dt = (DataTable)Page.Session["Files"];
        dt = (DataTable)ViewState["Files"];

        int cn = dt.Rows.Count;

        if (Convert.ToInt32(dt.Rows[recordToDelete].ItemArray[0]) != 0)
        {
            DeleteAttachment(Convert.ToInt32(dt.Rows[recordToDelete].ItemArray[0]), dt.Rows[recordToDelete].ItemArray[1].ToString());
        }

        dt.Rows.RemoveAt(recordToDelete);

        dt.AcceptChanges();
        //Page.Session["Files"] = dt;
        ViewState["Files"] = dt;
        dgdUpload.DataSource = dt;
        dgdUpload.DataBind();
    }
    private void AddMoreColumns()
    {

        dt = new DataTable("Files");

        dc = new DataColumn("ID", Type.GetType("System.Int16"));
        dt.Columns.Add(dc);

        dc = new DataColumn("FileName", Type.GetType("System.String"));
        dt.Columns.Add(dc);

        dc = new DataColumn("FilePath", Type.GetType("System.String"));
        dt.Columns.Add(dc);

        //Page.Session["Files"] = dt;
        ViewState["Files"] = dt;

    }
    private void AddRow(string file, string path, int ID, Boolean bolCheckForfiles)
    {

        Boolean bolAddRow = true;
        //dt = (DataTable)Page.Session["Files"];
        dt = (DataTable)ViewState["Files"];
      
        if (dt == null)
        {
            AddMoreColumns();

        }
        if (bolCheckForfiles)
        {
      
            if (dt.Rows.Count > 0)
            {
                foreach (DataRow drExistingrow in dt.Rows)
                {

                    if (drExistingrow["FileName"].ToString() == file)
                    {
                        bolAddRow = false;
                    }
                }
            }
        }
        if (bolAddRow)
        {

            dr = dt.NewRow();



            dr["ID"] = ID;
            dr["FileName"] = file;
            dr["FilePath"] = path;



            dt.Rows.Add(dr);

            //Page.Session["Files"] = dt;
            ViewState["Files"] = dt;

            dgdUpload.DataSource = dt;

            dgdUpload.DataBind();//bind in grid

        }
        else
        {
            lblMessage.Visible = true;
            lblMessage.Text = "Same File Name already exists!!!";
        }
    }

  
  

    protected void dgdUpload_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "View")
        {
            string file = e.CommandArgument.ToString();
            DownloadAttachment(file);

        }
    }



        if (dt != null)
        {

            int _dtcnt = dt.Rows.Count;
            foreach (DataRow dr in dt.Rows)
            {

                Boolean bolAddAttachment = true;
                fileName = dr["FileName"].ToString();
                if (itmCorrectiveActionFinding.Attachments.Count > 0)
                {
                    foreach (string strAttachedname in itmCorrectiveActionFinding.Attachments)
                    {

                        if (fileName == strAttachedname)
                        {
                            bolAddAttachment = false;

                        }
                    }
                }
                if (bolAddAttachment)
                {
                    string strFilepath = dr["FilePath"].ToString();

                    StreamReader sr = new StreamReader(strFilepath);

                    Stream fStream = sr.BaseStream;

                    contents = new byte[fStream.Length];

                    fStream.Read(contents, 0, (int)fStream.Length);

                    fStream.Close();

                    itmCorrectiveActionFinding.Attachments.Add(fileName, contents);

                    itmCorrectiveActionFinding.Update();
                    lstCorrectiveActionFinding.Update();
                    SPContext.Current.Web.Update();
                    System.IO.File.Delete(strFilepath);

                }
            }
        }