what is Sharepoint eventhandler? Synchronous and Asynchronous Events

SharePoint Event Handler is a program that enhances and adds functionality throughout SharePoint List, List Items or Sites. Event Feature can be deployed to new or existing sites by using Features. SharePoint Object Model provides several event classes that can target event handlers to List, List Item or Sites.

Synchronous and Asynchronous Events

In addition to asynchronous events, SharePoint Server 2007 introduces synchronous events i.e. events that activate before the action occurs. Synchronous events can trap an item, document library or site before it is deleted. Synchronous event removed the ability for users to delete an item from the document library or event restricts users to delete columns from the document library.

SharePoint Object Model exposes several event classes inherit from Microsoft.SharePoint assembly. There are three main event classes:

SPItemEventReceiver
SPListEventReceiver
SPWebEventReceiver

Each class includes both synchronous and asynchronous methods to work with Item, List or Web Level. In this article, I will demonstrate how hook an ItemDeleting, FieldDeleting or SiteDeleting Event to restrict users from deleting an item from the document library or restrict users from deleting a column from the document library.

SPItemEventReceiver

By overriding a member of SPItemEventReceiver class, developers can restrict users from deleting, updating an Item in a List. Some of the important SPItemEventReceiver class members are described below.

Event	Event Type	Description
ItemAdded	Asynchronous	After an Item Added in a list
ItemAdding	Synchronous	Before an Item Added in a list
ItemAttachmentAdded	Asynchronous	After an attachment added from a list item
ItemAttachmentAdding	Synchronous	Before an attachment added from a list item
ItemAttachmentDeleted	Asynchronous	After attachment deleted from a list item
ItemAttachmentDeleting	Synchronous	Before attachment deleted from a list item
ItemCheckedOut	Asynchronous	After an item checked-In in a list
ItemCheckingOut	Synchronous	Before an item is checked out in a list
ItemCheckedIn	Asynchronous	After an item is checked-In in a list
ItemCheckingIn	Synchronous	Before an item is checked-In in a list
ItemDeleted	Asynchronous	After an item is deleted from a list
ItemDeleting	Synchronous	Before an item is deleted from a list
ItemUpdated	Asynchronous	After an item is updated in a list
ItemUpdating	Synchronous	Before an item is updated in a list

For the demonstration, I am using a synchronous event i.e. ItemDeleting, which restrict users of deleting an item in a list (document library). You can invoke this event with any type of list of document library by specifying the ListTemplateId attribute in a Feature Schema file e.g. ItemEventReceiver.xml.

See below screen shot of ItemEventReceiver.cs and ItemEventReceiver.xml for defining the ListTemplateId and Assembly.

ItemEventReceiver.cs class

ItemEventReceiver.xml file

The following table shows default list template integer IDs that you can use in defining the template ID for event handlers.

List Template Id	List Template
100	Generic list
101	Document library
102	Survey
103	Links list
104	Announcements list
105	Contacts list
106	Events list
107	Tasks list
108	Discussion board
109	Picture library
110	Data sources
111	Site template gallery
113	Web Part gallery
114	List template gallery
115	XML Form library
120	Custom grid for a list
200	Meeting Series list
201	Meeting Agenda list
202	Meeting Attendees list
204	Meeting Decisions list
207	Meeting Objectives list
210	Meeting text box
211	Meeting Things To Bring list
212	Meeting Workspace Pages list
300	Portal Sites list
1100	Issue tracking
2002	Personal document library
2003	Private document library

SPListEventReceiver

By overriding a member of SPListEventReceiver class, developers can restrict users from deleting, updating a list from a Site. Some of the important SPListEventReceiver class members are described below.

Event	Event Type	Description
FileAdded	Asynchronous	After a document is added in a List (Document Library)
FileAdding	Synchronous	Before a document is added in a List (Document Library)
FileDeleted	Asynchronous	After a document is deleted from a list (Document Library)
FileDeleting	Synchronous	Before a document is deleted from a list (Document Library)
FileUpdated	Asynchronous	After a document is updated in a list (Document Library)
FileUpdating	Synchronous	Before a document is updated in a list (Document Library)

For the demonstration, I am using a synchronous event i.e. FieldDeleting, which restrict users of deleting a document from a list (Document Library). In this case, I am again specifying ListTemplateId = 101 for Document Library in a ListEventReceiver.xml file.

SPWebEventReceiver

By overriding a member of SPWebEventReceiver class, developers can restrict users from deleting a site from a site collection or even completely delete a site collection. Some of the important SPWebEventReceiver class members are described below.

Event	Event Type	Description
SiteDeleted	Asynchronous	After a site collection has been deleted.
SiteDeleting	Synchronous	Before a site collection is being deleted.
WebDeleted	Asynchronous	After a web site has been deleted.
WebDeleting	Synchronous	Before a web site is being deleted.

For the demonstration, I am using a synchronous event i.e. WebDeleting, which restrict users of deleting a web site from a site collection.

We need Visual Studio 2005 to create a solution for providing the required functionality.

The steps required to create event handler for SharePoint Sever application are described below:

Create a C#.NET Class Library Solution in Visual Studio .NET 2005 and give a name, such as EventHandlerFeature.
On the Project Menu, choose Add Reference to Open Reference dialog box.
Scroll down to Windows SharePoint Services and click the references the Microsoft.SharePoint.dll.
Create a new folder inside Project Solution and give a name such as "Features".
Create three folders inside Features folder and give names such as ItemEventReceiver, ListEventReceiver and WebEventReceiver folders.
Create Feature.XML and feature schema file for ItemEventReceiver folder.

Attributes of Feature.XML File

Some of the attributes of Feature Tag are described below:

Attribute	Value	Description
ID	GUID	Contains the globally unique identifier (GUID) for the Feature.
Title	Text	Returns the title of the Feature. Limited to 255 characters.
Scope	Farm/WebApplication/Site/Web	Can contain one of the following values: Farm (farm), WebApplication (Web application), Site (site collection), and Web (Web site).
Hidden	True/False	This attribute equals FALSE by default.
AlwaysForceInstall	True/False	Optional Boolean. TRUE if the Feature is installed by force during installation even if the Feature is already installed. For example, if set to TRUE, Feature installation callouts will always fire anytime a user tries to install the Feature (even if it is already installed) by using either the scanforfeatures or installfeature command-line operation. This attribute equals FALSE by default. The AlwaysForceInstall attribute affects Features of all scopes.

See screen shot below of Feature.XML file.

Repeat above step from ListEventReceiver and WebEventReceiver folders.
Create a new C#.NET class and give a name, such as ItemEventReceiver.cs class. Add Microsoft.SharePoint namespace and inherits this class from SPItemEventReceiver class in order to overrides ItemDeleting member for restricting users from deleting an Item from a list or document library.
Repeat above step and create two more classes and give names, such as ListEventReceiver.cs and WebEventReceiver.cs.
Build the Solution
Sign the solution by using SN utility of .NET command prompt or specifying the Sign Assembly name in the solution properties.

Note: Complete source code of solution and package file is attached with this article.