After tracking the DataGridRow.Item and DataGridRow.IsNewItem properties, I discover that: each added item (to DataGrid when Source is ObservableCollection<MyClass>),
IsNewItem Always positive, Although Item Although he is not a NewItemPlaceholder.
Afterwards I looked at MSDN and saw that it was indeed affected by two factors:
Gets or sets a value that indicates whether the DataGridRow is a placeholder for a new item or for an item that has not been committed.
How do I committ added item?