Boxing and Unboxing in C#

What is  Boxing and Unboxing in .Net?

Boxing- Conversion of value types to reference types.

E.g.-                int i=1;

                        object obj= i;

Unboxing- Conversion of reference types to value types.

E.g.-               int j= (int) obj;

Basically we should avoid boxing and unboxing in practical implementation as it degrades the performance.