这节课我们主要学习了class和object，也稍微介绍了一些继承和多态。
首先我按照课堂上讲的写了deep copy 算是this的引入▼
之后就到this了，我们讲了this指针的五个用法：
1. To qualify instance members otherwise hidden by parameters.
示例如下▼
2. To pass the current object as a parameter to another method.
3. With Indexers.
4. To call one overloaded constructor from another.
示例如下▼
5. To explicitly invoke methods and members of a class.
然后是Built in type: String▼
最后，我尝试了Readonly Field▼
