Introduction: Interfaces are an important aspect of object-oriented programming in C#. An interface defines a contract that specifies the members that a class must implement. This means that an interface defines what methods, properties, events, or indexers a class must provide, but does not provide the implementation for those members. Example: Here's an example of how you can use an interface in C#: using System; interface ICalculator { int Add ( int a, int b ) ; int Subtract ( int a, int b ) ; int Multiply ( int a, int b ) ; int Divide ( int a, int b ) ; } class BasicCalculator : ICalculator { public int Add ( int a, int b ) { return a + b; } public int Subtract ( int a, int b ) ...