在介绍本章之前，我们先看一下C#的编译过程，如下图所示：
图片摘自：http://www.developingthefuture.net/compilation-process-and-jit-compiler/
The process of compilation in a managed environment (.Net, Java)
The process of compilation in managed environments is slightly different. Let's take .Net and Java as examples. The code we write in our favourite IDE is first checked by the IDE itself. Then, when compiled into object files and linked into dynamic/static libraries or executables, it is checked again, and finally it is checked at runtime. One common characteristic of the managed environments is that the compiler does not produce binary code, but rather intermediate meta-code, called MSIL - Microsoft Intermediate Language in .Net and Bytecode in Java.
After that, the MSIL is translated to binary code at runtime by the JIT (Just In Time) compiler, meaning that the code you write is only interpreted when it is actually used. This allows the CLR (Common Language Runtime) to precompile and optimize your code, achieving improved performance, at the cost of increased startup time. But you can also precompile your application using Ngen (Native Image Generator) to speed up the startup, without the benefits of runtime optimization.
C#是静态类型的语言，需要编译，所以我在使用变量前必须告诉编译器变量的类型。我们使用如下所示的语法定义变量：
int a = 1; //定义一个整数类型的变量a float b = 1f;//定义一个float类型的变量b char c= 'a';//定义一个字符类型的变量c string str= "gavinsun";//定义一个字符串类型的变量str bool isTrue= true;//定义一个bool类型的变量isTrue