Definition: Event Receiver enables us to apply custom code against any event/action on specific SharePoint object. Event Receiver is a method that is called when any triggering action occurs on specific SharePoint object.

Types of Event Receivers: There are two types of event receivers.

(1) Synchronous event receivers:

• They are known as ‘Before’ events
• Synchronous event receivers are called before the actual event occurs and before SharePoint has written any content in Content Database, that’s why they are called before events
• Events ending with “–ing” are named synchronous events. E.g. Item Adding, Item Deleting
• Event is executed in the same thread as the triggering action is running.
• You can cancel the running event
• You can get HttpContext
• You can display error message

(2) Asynchronous event receivers:

• They are known as ‘After’ events
• Asynchronous event receivers are called after the actual event occurs and after SharePoint has written content in Content Database, that’s why they are called after events
• Events ending with “–ed” are named asynchronous events. E.g. Item Added, Item Deleted
• Event is executed in different thread than the triggering action.
• You cannot cancel the running event
• You cannot get HttpContext
• You cannot display error message

Event Receivers Level: Below are the event receivers - execution levels

• Site(Site Collection) Events
• Web Events
• List Events
• List Field Events
• Item Events
• Feature Events
• Email events