You are going to need to learn things your entire career. It's more than just syntax to relearn though. It is libraries and the like as well.
However, it is a paid opportunity to gain experience in something else so I fail to see the problem. .net is used in industry. It isn't as if they have you using a proprietary language nobody else uses.
I can think of two concerns here that the original poster might be thinking about:
the person being switched has a bunch of experience in Java and is concerned about splitting beteen the two stacks on his/her resume - I think this is ok. If you want your next job to be in Java, work on side projects in Java to keep it fresh.
the person is new to working and wants his/her first job to be in Java - Why would it matter which language one starts with? I think this is ok too, I just understand the domain less.
In college I knew a few equally well. My first job that involved coding was in Java. Which wound up taking me down the path I am on today. It could have easily been a different path though. And I work with many people who used to program in something else.
Vishal Hegde wrote:ofcourse it is you study in java domain and suddently you are in .net again you will need to learn the syntax etc
In the field of software development you are going to unlearn and learn quite a lot of things. So this isn't a surprise. If you are working on C#, you are still in a familiar territory. People have been moving into totally different programming paradigm like Object oriented to Functional. Or may be from statically typed to a dynamically typed language.