C# 什么是表达式树，如何使用它们，为什么要使用它们？
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What are Expression Trees, how do you use them, and why would you use them?
提问by Donny
I just came across the concept of expression trees which I have heard multiple times. I just want to understand what is meant by an expression tree and its purpose.
我刚刚遇到了我多次听到的表达树的概念。我只想了解表达式树的含义及其目的。
I would love it if someone could also direct me to simple explanations and samples of use.
如果有人也可以指导我进行简单的解释和使用示例，我会很高兴的。
采纳答案by Roy Dictus
An Expression Tree is a data structure that contains Expressions, which is basically code. So it is a tree structure that represents a calculation you may make in code. These pieces of code can then be executed by "running" the expression tree over a set of data.
表达式树是一种包含表达式的数据结构，它基本上是代码。所以它是一个树结构，表示您可以在代码中进行的计算。然后可以通过在一组数据上“运行”表达式树来执行这些代码段。
A great thing about expression trees is that you can build them up in code; that is, you build executable code (or a sequence of steps) in code. You can also modify the code before you execute it by replacing expressions by other expressions.
表达式树的一大优点是您可以在代码中构建它们；也就是说，您在 code 中构建可执行代码（或一系列步骤）。您还可以在执行之前修改代码，方法是用其他表达式替换表达式。
An Expression is then a function delegate, such as (int x => return x * x).
一个表达式则是一个函数委托，例如(int x => return x * x).
See also http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx
另见http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx
