Is it mandatory to have ArcEngine Licence to learn ArcObjects?I was able to develop Addins for arcmap but I need in-depth knowledge of ArcObject so started exploring MapControl Application but it started flashing message boxes for using Unlicensed control.If ArcEngine is must then is there any trail version of Arcengine 10.2 Licence available for learning.Is there are any online manual to learn ArcObjects.
You need to license ArcGIS Engine which gives you interface for interacting with ArcObjects components. Your option is to get EDN license for development-test from your local Esri distributor. This is the most common way to get started ArcGIS Desktop development. Here is the list of what you get with EDN.
To learn ArcObjects, you might like a new book published recently: Beginning ArcGIS for Desktop Development using .NET. Esri Help is very comprehensive and has multiple tutorials too.
PS. Evaluate your needs carefully to see if you really need to use ArcObjects. It has a slightly steep learning curve and there isn't very much new happening in this area. There is a good chance that in several years you will end up with code that will be hard to maintain and develop further. If you really need a customized Desktop application, then yes, ArcObjects is the only platform (apart from my favourite MapObjects /old days/) applicable. Otherwise, consider developing a web based application by using ArcGIS Server REST API + Web Mapping API (JS); ArcGIS web APIs have become much more powerful over last years and you will be amazed how much of desktop application functionality can be moved into a web-based solution.
Esri is investing into Python, so you might like taking a look at Python toolboxes, Python add-ins and Python script tools. If you are comfortable with Python libraries, consider using wxPython or Kivy when you need a GUI and need to wrap some ArcGIS functionality (I've succeeded in wrapping arcpy functions within both of them).