My solution:
Save entity when DataGrid's SelectedChanged event happen, like this:
<br />
Grid.SelectedChanged+=(s,e)=><br />
{<br />
};<br />
The entity to be save is current DataGrid's SelectItem,but it have already changed when SelectedChanged event happend,so I have to do anther thing to insure the to save entity is the previous SelectItem of Grid.