This is one of my favorite question. Since String is one of the most important type in programming, you expect lot of question related to String any data structure interview. There are many ways to reverse Sting in C# or any other programming language, and interviewer will force you to solve this problem by using without API i.e. without using reverse() method of StringBuffer. In follow-up he may ask to reverse String using recursion as well.
static void Main(string args)
string strHellowWorld = "Hellow World!";
char arrHellowWorld= strHellowWorld.ToCharArray();
string strRevers = "";
for (int i = arrHellowWorld.Length-1; i >= 0; i--)
strRevers += arrHellowWorld[i].ToString();