C# Lambda 到表达式树的转换
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Lambda to Expression tree conversion
提问by Prashant Cholachagudda
I will keep it really simple,
我会保持非常简单，
How do I get expression tree out of lambda??
如何从 lambda 中获取表达式树？
or from query expression ?
还是来自查询表达式？
采纳答案by Konrad Rudolph
You must assign the lambda to a different type:
您必须将 lambda 分配给不同的类型：
// Gives you a delegate:
Func<int, int> f = x => x * 2;
// Gives you an expression tree:
Expression<Func<int, int>> g = x => x * 2;
The same goes for method arguments. However, once you've assigned such a lambda expression to a Func<>type, you can't get the expression tree back.
方法参数也是如此。但是，一旦将这样的 lambda 表达式分配给Func<>类型，就无法取回表达式树。
回答by Pierre Arnaud
Konrad's reply is exact. You need to assign the lambda expression to Expression<Func<...>>in order for the compiler to generate the expression tree. If you get a lambda as a Func<...>, Action<...>or other delegate type, all you have is a bunch of IL instructions.
康拉德的回答是准确的。您需要将 lambda 表达式分配给以Expression<Func<...>>让编译器生成表达式树。如果你得到一个lambda作为Func<...>，Action<...>或其他委托类型，你已经是一堆的IL指令。
If you really need to be able to convert an IL-compiled lambda back into an expression tree, you'd have to decompile it (e.g. do what Lutz Roeder's Reflector tool does). I'd suggest having a look at the Cecillibrary, which provides advanced IL manipulation support and could save you quite some time.
如果您确实需要能够将 IL 编译的 lambda 转换回表达式树，则必须对其进行反编译（例如，执行 Lutz Roeder 的 Reflector 工具所做的工作）。我建议看看Cecil库，它提供了高级 IL 操作支持，可以为您节省相当多的时间。
回答by joniba
Just to expand on Konrad's answer, and to correct Pierre, you can still generate an Expression from an IL-compiled lambda, though it's not terribly elegant. Augmenting Konrad's example:
只是为了扩展 Konrad 的答案并纠正 Pierre，您仍然可以从 IL 编译的 lambda 生成表达式，尽管它不是非常优雅。增强康拉德的例子：
// Gives you a lambda:
Func<int, int> f = x => x * 2;
// Gives you an expression tree:
Expression<Func<int, int>> g = x => f(x);
