In itemupdating event
There is a difference between the behaviour of Before and After properties when dealing with document libraries versus when dealing with lists.For documents, Before and After properties are guaranteed for post events, such as Item Updated, but Before properties are not available for post events on list items.
With the help of this you can find your solution according to your requirement.The most common set of receivers used, however, are part of SPItem Event Receiver which let you wire your code up to a number of events that can occur to items on a list or library.When working with events, you’ll quickly find that before (synchronous) and after (asynchronous) events exist, and the method suffix such as “ing” (e.g. Item Added) will tell you whether it gets invoked before or after the actual change is made. And, as you get deeper, you’ll even find that you can extract the before and after state of the change.How can we prevent a user from changing a certain column for an item in a list event?
From the list table, you can see if we hook into the Item Updating event, we can compare the current item’s value (properties.
New value means that the correct value for the column was available.