2.Select: class library-name: jisuanqiDll.

3. Write a calculator parent class with its own attributes, abstract methods and constructors.

4. Write an additional class and inherit the override method and constructor of the parent class.

5. Write a console application test and add a reference to JisuanqiDll.

6. After referencing JisuanqiDLL, you can use the methods and properties in this class. Add and subtract with a switch cycle, and then judge whether the user input correctly outputs the result. Enter an operator that is not in the program. Tip: This function is not available.

7. Test multiplication and division.