zoukankan      html  css  js  c++  java
  • const VS readonly in detail

      We know both const and readonly can be used to define unchangeable variables,but what is the difference between'em since they are so similar to each other?

      There are a few points below:

      1.const variables must have a certain value in the compilation time, but for readonly variables, its value can be assigned in run time, for example,
    we can define a readonly variable and assign its value by this way:

        class foo
        {
            public readonly int a;
            public foo()
            {
                a = 10;
            }
        }
    
        or maybe this:
    
        class foo
        {
            public static readonly int a;
            static foo()
            {
                a = 10;
            }
        }

    As we can see, we can assign value to a readonly variable in the constructor,but you won't do this to a const variable.
    BTW: a class may have a static constructor, check it out on google

      2.basically, "const" is just simply a shortcut for "find & replace" when our code is being compiled.Think about this: we have two assamblies say ass1 and ass2, and there is a public const variable,for example "public const int a=10" in some class of ass1 while this const variable is used in another class of ass2. At first we compile these two assamblies and run it, we got the value of the const variable  and that certainly is 10. But now we change the value into 100, and recompile the ass1,guess what we got for the value of the const variable “a” ? 100? No,it's still 10. Why? Because the const variable "a" is replaced into "10" by "find & replace" when the ass2 being compiled, hence, if you don't recompile it, it will keep the value of "10" forever no matter the variable is changed or not in the ass1 where it is defined.
      But this situation won't happend to readonly variables, because the value of readonly variables "is settled down in run time" and it's not "find & replace", it's always referenced by ass2, so the ass2 can always get the newest value as soon as it changed in ass1.

      3.const variables are implicitly static and we can not put the keyword "static" on them, but readonly variables can be done so.We can use "ClassName.ConstName" notation to use public const variables in a class just like readonly variables which are static,if a readonly variable is NOT static, we can only use it by the instance of the class owning it.

  • 相关阅读:
    Android-WebView路由登录192.168.1.1
    Win7 & VirtualBox虚拟Ubuntu 本地虚拟机之间文件共享
    Android 简单的JNI编程
    Android ActionBar简单使用
    多个APK之间简单数据共享
    js代码移动Div 移动平台与PC平台
    JavaScript面向对象
    《SSO CAS单点系列》之 APP原生应用如何访问CAS认证中心
    insh.exe:*** Couldn't reserve space for cygwin's heap,Win32 error 0
    解决:SSM框架中普通类调用Service的问题 (转)
  • 原文地址:https://www.cnblogs.com/lwhkdash/p/2973609.html
Copyright © 2011-2022 走看看