Event Receivers in SharePoint Theory



Event Receivers in SharePoint

Through this article I am trying to explain about event receivers in SharePoint.

An event receiver is a class that contains one or more methods known as event handlers that are executed automatically by SharePoint in response to events such as adding item to a list. 
You can use event handlers for data validation, data integrity etc.

SharePoint categories events into of two types 1. Before events (synchronous events) 2. After events (asynchronous events).

1. Before events (synchronous events):
These events fires before an action occurs and before SharePoint has written any data to the content database.
For example ItemAdding event will fire when trying to add an item to a list. These events does support cancelling of event action. These events are always end with 'ing' like FieldAdding, ItemAdding, ItemUpdating, WebAdding, WebDeleting etc. These events are also know as synchronous events.

2. After events (asynchronous events):
These events fire after the event action has completed and after SharePoint Foundation has written to the content database to commit the event action. These events does not support cancelling of event action. These events does support cancelling of event action. These events are always end with 'ed' like FieldAdded, ItemAdded, ItemUpdated, WebAdded, WebDeleted etc. These events are also know as asynchronous events.

Events, Event Receivers & Object Model:

Event receivers are nothing but custom classes which are derived from event receiver base classes that are defined by Microsoft in Microsoft.SharePoint assembly.

Some of the base classes are 
SPItemEventReceiver: There are related to SharePoint items.
SPListEventReceiver: Related to lists.
SPWebEventReceiver: Related to webs.
SPWorkflowEventReceiver: Related to workflows instances. etc.

Remember all these classes(except SPEmailEventReceiver and SPFeatureReceiver) are derived from a common base class know as SPEventReceiverBase.

Here there are different event receivers:
Item-Level Event Receivers:
The base class for this type of event receivers are SPItemEventReceiver and are related to SPListItem instances.

Some events are: ItemAdded, ItemAdding, ItemCheckedIn, ItemCheckingIn, ItemCheckedOut, ItemCheckingOut, ItemDeleted, ItemDeleting etc.

List-Level Event Receivers:
The base class for this type of event receivers are SPListEventReceiver and events are related to lists. These events fires whenever any changes to the fields of an existing list as well as adding or deleting list instances.

Some events are: FieldAdded, FieldAdding, FieldDeleted, FieldDeleting, FieldUpdated, FieldUpdating, ListAdded, ListAdding, ListDeleted, ListDeleting etc.

Web-Level Event Receivers:
These are events related to Site Collection deletion and website creation, deletion, moving, and provisioning. The base class for these type of event receiver is SPWebEventReceiver.

Some events are: SiteDeleted, SiteDeleting, WebAdding, WebDeleted, WebDeleting, WebProvisioned etc.

Workflow Event Receivers:
These event receivers are for running workflows and the base class for these type of event receivers are: SPWorkflowEventReceiver.

Some events are: WorkflowStarting, WorkflowStarted, WorkflowCompleted etc.

E-Mail Event Receivers:
These event receivers are for e-mail–enabled list instances and the base class is SPEmailEventReceiver.
For this type of event receivers you have to override the EmailReceived method.

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