By reference to Josh Smith's 'Drag and Drop Items in a WPF ListView'(http://www.codeproject.com/KB/WPF/ListViewDragDropManager.aspx),
I implement my version of DataGridDragDropManager, makes DataGrid support draging one DataGridRow from one row to another row.
Anyone who want make datagrid support row drag and drop can refernce to Josh Smith's code.
Only a few modification need be made to port it from for ListView to for DataGrid.
I hope DataGrid can support reorder row by drag and drop in future.