CitiusTech Senior Software Engineer interview questions

I was applying for Senior Software Engineer in .NET. There were 3 rounds of interview. 2 technical and 1 hr. 1st round focussed more on basics. They gave a piece of code and asked OOPS concepts applied on it. The code was not complete so he asked me to complete it. Since i had experience with LINQ and WCF. He asked me to write code for a problem statement using LINQ. He asked me basics of WCF too like why was it introduced and what problem it solves. 2nd round was intermediate level with few advanced topics. He asked me what was i good at. He asked questions on polymorphism. He asked me about design patterns and he told me to write code for singleton. He also asked a question on Task Parallel library. Finally he touched upon SOLID design principles. He asked me to explain anyone of them with code example. He asked why do you need these principles when we already have OOPS. Finally HR round took place where salary expectations were set. I accepted the offer since it was within my expectations