动态（C＃4）和var有什么区别？ [英] What's the difference between dynamic (C# 4) and var?

问题描述

我已经阅读了很多有关C＃v4附带的新关键字的文章，但我看不出动态和动态之间的区别。

I had read a ton of articles about that new keyword that is shipping with C# v4, but I couldn't make out the difference between a "dynamic" and "var".

这篇文章让我考虑了一下，但我仍然无法考虑看到任何区别。

This article made me think about it, but I still can't see any difference.

难道您只能将 var用作局部变量，而可以同时用作局部变量和全局变量吗？

Is it that you can use "var" only as a local variable, but dynamic as both local and global?

您可以显示一些没有动态关键字的代码，然后再显示具有动态关键字的相同代码吗？

Could you show some code without dynamic keyword and then show the same code with dynamic keyword?

推荐答案

var 是静态类型的-编译器和运行时知道类型-它们只是为您节省了一些键入内容...以下是100％相同的：

var is static typed - the compiler and runtime know the type - they just save you some typing... the following are 100% identical:

var s = "abc";
Console.WriteLine(s.Length);

和

string s = "abc";
Console.WriteLine(s.Length);

所有发生的事情是编译器认为 s 必须是字符串（来自初始化程序）。在这两种情况下，它都知道（在IL中） s.Length 表示（实例） string.Length 属性

All that happened was that the compiler figured out that s must be a string (from the initializer). In both cases, it knows (in the IL) that s.Length means the (instance) string.Length property.

动态是非常不同的野兽；它与 object 最相似，但是具有动态分派：

dynamic is a very different beast; it is most similar to object, but with dynamic dispatch:

dynamic s = "abc";
Console.WriteLine(s.Length);

在这里，输入 s 保持动态。它不知道 string.Length ，因为它不知道关于 s c>在编译时。例如，以下代码也将编译（但不运行）：

Here, s is typed as dynamic. It doesn't know about string.Length, because it doesn't know anything about s at compile time. For example, the following would compile (but not run) too:

dynamic s = "abc";
Console.WriteLine(s.FlibbleBananaSnowball);

在运行时（仅），它将对 FlibbleBananaSnowball 属性-找不到它，并在火花中爆炸。

At runtime (only), it would check for the FlibbleBananaSnowball property - fail to find it, and explode in a shower of sparks.

具有 dynamic ，属性/方法/运算符/等根据实际对象在运行时解析。与COM（可以具有仅运行时属性），DLR或其他动态系统（例如 javascript 。

With dynamic, properties / methods / operators / etc are resolved at runtime, based on the actual object. Very handy for talking to COM (which can have runtime-only properties), the DLR, or other dynamic systems, like javascript.

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