The ICancelAddNew Interface: Supporting Transactional Inserts in a Collection in .NET

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The ICancelAddNew Interface: Supporting Transactional Inserts in a Collection
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The ICancelAddNew interface lets you add and edit new items to a collection with the item in a transient state, which allows you to remove the new item before finalizing the addition if desired (if a problem occurs while initializing the object's properties) This interface was added in NET 20 to break an implicit coupling that occurs with the IEditableObject interface when adding new items to a collection Prior to adding this interface, objects in the collection that implemented the IEditableObject interface had to notify their parent collection if editing was canceled on a newly added item so the collection could remove that item from the collection With ICancelAddNew , the collection can take care of the removal, and the contained object no longer has to have a direct coupling to its containing collection for backing out a newly added item if initialization is canceled Another capability that can come in handy for a richer data-binding experience is to support transactional adding of items to the collection To understand this, consider a data table bound to a grid The grid presents a blank line at the bottom of the grid that lets you add new rows to the table But what if there are constraints on the columns of that row or if there is validation that needs to occur based on the input to multiple fields before the item should be added to the data collection How can you prevent inconsistent data from being added to the collection After all, there needs to be an object instance somewhere to accept the data being input by the user as they tab from field to field A new row in the data source is the most logical kind of object to create But you don't want to actually add the object until the addition is considered "complete"whatever that means based on the collection and the data objects that go within it To support this scenario, the ICancelAddNew interface has been defined to allow a collection to decide whether to accept or reject a new item that has been added to the collection through this interface's methods If a collection supports transactional adding of items to the collection, it should implement the ICancelAddNew interface Bound controls can then callEndNew(int index) to commit the transaction of adding a new item or CancelNew(int index) to roll back the addition This lets the control callAddNew on the list, get a new item back, and start setting values on the new object If the code calls CancelNew with the index of the item that was added, the new object can be discarded without actually adding it to the collection for good If the code calls EndNew with the index or performs any other operation on the collection, the addition should be committed The object itself never needs to know about its transient state with respect to membership in the collection; that is all handled by the collection itself This is a little different behavior than what you might expect In the world of distributed and database transactions, you are expected to explicitly commit the transaction or it should roll back In the case of ICancelAddNew, committing is the default behavior even if EndNew isn't explicitly called based on the contract specified by the Framework So inserting or removing other items, or setting the current item to another item in the collection, is considered to take the focus off the item being added and will commit the item to the collection Both the BindingList<T> class that will be discussed in detail in 9 and the BindingSource class implement this interface for the collections they manage Because the BindingList<T> generic class cannot know what to do about the transactional semantics of a call to AddNew or CancelNew, you will need to derive a class fromBindingList<T> and override the base class methodsAddNewCore and CancelNewCore to provide the implementation that makes sense for your scenario
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