怎么把Datagrid中的内容保存成.txt

private void save_Click(object sender, RoutedEventArgs e)
{
SaveFileDialog savedialog = new SaveFileDialog();
savedialog.Filter = "*.txt|*.txt";
if (savedialog.ShowDialog(this) == true)
{
StreamWriter sw = new StreamWriter(savedialog.FileName);
sw.Write(DG1.Columns);//此处保存后文件里为何没有内容？
}
}
DG1是表格的name，怎么把DataGrid中的每行每列数据保存成文本？求高手老师教我！1