如何在“添加”按钮单击时将行添加到WPF dataGrid？

Here am using the below code for adding rows to wpf datagrid,but if am again click to add means the row cannot be added and previous row replaced by new added details....So how to make possible to add new more rows based on add button click externally. Here the code a using ,its working for one row only added in datagrid,So how to make more new adding rows with externally button click.Here am using the below code for adding rows to