Time out error while operating FtpWebResponse objects

I have some recursive code to operate FtpWebResponse objects, but it always return an error in the recursion.

private void DownloadFolder(string _strFolderName)
        {
            string strFolderURL = this.strFTPURL + "/" + _strFolderName;
            try
            {
                StringBuilder result = new StringBuilder();
                StreamReader reader = null;
                FtpWebRequest listRequest =
                    (FtpWebRequest)WebRequest.Create(strFolderURL);
                listRequest.Method = WebRequestMethods.Ftp.ListDirectoryDetails;
                FtpWebResponse listResponse =
                    (FtpWebResponse)listRequest.GetResponse();
                reader = new StreamReader(listResponse.GetResponseStream());
                    //directories
                    if(IsDirectory)
                    {
                            this.DownloadFolder(_strFolderName + "/" + strInfo[strInfo.Length - 1]);
                    }
                    //files
                    else
                    {
                            this.DownloadFile(_strFolderName, strInfo[strInfo.Length - 1]);
                    }
                }
                reader.Close();
                listResponse.Close();
            }
            catch (Exception ex)
            {
                System.Windows.Forms.MessageBox.Show(ex.Message);
            }
        }

Error: The operation has timed out.

 

Solution:

System.Net.ServicePointManager.DefaultConnectionLimit = 100; //any number you like

Causation:
While running into recursion, the FtpWebResponse object in parent function is still unclosed, the connection become more and more, but its default value is 2 only like below:

        //
        // Summary:
        //     Gets or sets the maximum number of concurrent connections allowed by a System.Net.ServicePoint
        //     object.
        //
        // Returns:
        //     The maximum number of concurrent connections allowed by a System.Net.ServicePoint
        //     object. The default value is 2.
        //
        // Exceptions:
        //   System.ArgumentOutOfRangeException:
        //     System.Net.ServicePointManager.DefaultConnectionLimit is less than or equal
        //     to 0.
        public static int DefaultConnectionLimit { get; set; }