我的文件导出是不是写的有问题！望高手检查一下哪有问题

如果第二次导出同一文件夹下会在原有的文本下面继续添加，而且选择桌面路劲导出不了   
private void button5_Click(object sender, EventArgs e)
        {
            FolderBrowserDialog dialog = new FolderBrowserDialog();
            dialog.Description = "请选择文件路径";
            if (dialog.ShowDialog() == DialogResult.OK)
            {
                string foldPath = dialog.SelectedPath;
                string path = @foldPath + "PosDesBack.CSV";
                StreamWriter sw = new StreamWriter(path, true, Encoding.Default);
                foreach (PosDes pd in _posdesList)
                {
                    sw.WriteLine(string.Format("{0},{1},{2},{3}", pd.PosID, pd.Longitude, pd.Latitude, pd.PosDes));
                }
                sw.Close();
                MessageBox.Show("导出完成");
            }
        }