why chooseC # when there is java?

C# was designed 5 or 6 years after Java and, although it's a similar language in many ways, was therefore able to address perceived shortcomings in the latter..

Although performance has been similar, C# has therefore had more modern features than Java throughout its history (Java doesn't even have properties!) and this, IMO, has made it a more attractive proposition for building all kinds of applications.

However, since Sun Microsystems (the original inventor of Java) was taken over by Oracle, Java has started to catch up though whether it will ever do so completely is open to doubt. 

Java still has an advantage in being available for virtually all platforms and, whilst Microsoft C# has been mainly confined to Windows, Mono and Xamarin have effectively extended the language's reach to cover other major platforms as well.