C# 有没有一种简单的方法可以将（lambda 表达式）字符串解析为 Action 委托？
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Is there an easy way to parse a (lambda expression) string into an Action delegate?
提问by Whisk
I have a method that alters an "Account" object based on the action delegate passed into it:
我有一个方法可以根据传递给它的动作委托来更改“帐户”对象：
public static void AlterAccount(string AccountID, Action<Account> AccountAction) {
Account someAccount = accountRepository.GetAccount(AccountID);
AccountAction.Invoke(someAccount);
someAccount.Save();
}
This works as intended...
这按预期工作......
AlterAccount("Account1234", a => a.Enabled = false);
...but now what I'd like to try and do is have a method like this:
...但现在我想尝试做的是有一个这样的方法：
public static void AlterAccount(string AccountID, string AccountActionText) {
Account someAccount = accountRepository.GetAccount(AccountID);
Action<Account> AccountAction = MagicLibrary.ConvertMagically<Action<Account>>(AccountActionText);
AccountAction.Invoke(someAccount);
someAccount.Save();
}
It can then be used like:
然后可以像这样使用它：
AlterAccount("Account1234", "a => a.Enabled = false");
to disable account "Account1234".
禁用帐户“Account1234”。
I've had a look at the linq dynamic query library, which seems to do more or less what I want but for Func type delegates, and my knowledge of Expression trees etc isn't quite good enough to work out how to achieve what I want.
我看过linq 动态查询库，它似乎或多或少地做了我想要的，但对于 Func 类型的委托，我对表达式树等的了解还不够好，无法弄清楚如何实现我想要的想。
Is there an easy way to do what I want, or do I need to learn expressions properly and write a load of code?
有没有一种简单的方法可以做我想做的事，或者我是否需要正确学习表达式并编写大量代码？
(The reason I want to do this is to allow an easy way of bulk updating account objects from a powershell script where the user can specify a lambda expression to perform the changes.)
（我想这样做的原因是允许从 PowerShell 脚本批量更新帐户对象的简单方法，用户可以在其中指定 lambda 表达式来执行更改。）
回答by RossFabricant
There is no general way to parse a string into a lambda expression without a full compilation, because lambda expressions can reference things that are defined outside the lambda expression. I know of no library that handles the specific case you want. There's a long discussion of this on a threadon a C# discussion group.
如果没有完整的编译，没有将字符串解析为 lambda 表达式的通用方法，因为 lambda 表达式可以引用在 lambda 表达式之外定义的内容。我知道没有库可以处理您想要的特定情况。在C# 讨论组的一个线程上对此进行了长时间的讨论。
The easiest way to get what you want is to compile a method at runtime. You can write a function that takes in the string "a.Enabled = true; return a;" and sticks that in the middle of a function that takes an Account as a parameter. I would use this libraryas a starting point, but you can also use the function mentioned on another thread.
获得所需内容的最简单方法是在运行时编译方法。您可以编写一个接受字符串 "a.Enabled = true; return a;" 的函数 并将其粘贴在将 Account 作为参数的函数的中间。我会使用这个库作为起点，但您也可以使用另一个线程中提到的函数。
回答by MichaelGG
The Dynamic LINQ library is a fine choice, as it'll generate expressions you can compile to code in a lightweight fashion.
动态 LINQ 库是一个不错的选择，因为它会生成表达式，您可以以轻量级的方式编译为代码。
The example you provided actually produces a boolean -- so you should be able to ask for a Func and it might sort it out.
您提供的示例实际上生成了一个布尔值——所以您应该能够要求一个 Func 并且它可能会解决它。
Edit: This of course is wrong, as Expressions don't have assignment in them at all.
编辑：这当然是错误的，因为表达式中根本没有赋值。
So, another potential way is to take two lambdas. One to find the property you want, one to provide a value:
因此，另一种可能的方法是采用两个 lambda。一个是找到你想要的属性，一个是提供一个值：
(a => a.AccountId), (a => true)
(a => a.AccountId), (a => true)
Then use reflection to set the property referenced in the first lambda with the result of the second one. Hackish, but it's still probably lightweight compared to invoking the C# compiler.
然后使用反射将第一个 lambda 中引用的属性设置为第二个的结果。Hackish，但与调用 C# 编译器相比，它仍然可能是轻量级的。
This way you don't have to do much codegen yourself - the expressions you get will contain most everything you need.
这样你就不必自己做太多的代码生成——你得到的表达式将包含你需要的大部分内容。
回答by Alex Yakunin
That's easy:
这很容易：
- Use CodeDomto generate the module containing the "surrounding class" you'll use to build the expression; this class must implement the interface known to your application
- Use CodeSnippedExpressionto inject the expression into its member.
- Use Activatortype to create the instance of this class in runtime.
- 使用CodeDom生成包含您将用于构建表达式的“周围类”的模块；此类必须实现您的应用程序已知的接口
- 使用CodeSnippedExpression将表达式注入到其成员中。
- 使用Activator类型在运行时创建此类的实例。
Basically, you need to build the following class with CodeDom:
基本上，您需要使用 CodeDom 构建以下类：
using System;
using MyNamespace1;
using ...
using MyNamespace[N];
namespace MyNamespace.GeneratedTypes
{
public class ExpressionContainer[M] : IHasAccountAction
{
public Action<Account> AccountAction {
get {
return [CodeSnippedExpression must be used here];
}
}
}
}
Assuming that IHasAccountActionis:
假设IHasAccountAction是：
public IHasAccountAction {
public Action<Account> AccountAction { get; }
}
If this is done, you can get the expression compiled from string with ease. If you need its expression tree representation, use Expression<Action<Account>>instead of Action<Account>in generated type.
如果这样做，您可以轻松获得从字符串编译的表达式。如果您需要其表达式树表示，请使用Expression<Action<Account>>而不是Action<Account>在生成的类型中。
回答by jpbochi
You may try this: Dynamic Lambda Expressions Using An Isolated AppDomain
你可以试试这个：Dynamic Lambda Expressions Using An Independent AppDomain
It compiles a lambda expression using CodeDOM compiler. In order to dispose the in-memory assembly that gets created, the compiler runs on an isolated AppDomain. For the passing the expression through the domain boundary, it has to be serialized. Alas, Expression<>is not Serializable. So, a trick has to be used. All the details are explained in the post.
它使用 CodeDOM 编译器编译 lambda 表达式。为了处理创建的内存中程序集，编译器在隔离的AppDomain. 为了通过域边界传递表达式，它必须被序列化。唉，Expression<>不是Serializable。所以，必须使用一个技巧。所有细节都在帖子中解释。
I'm the author of that component, by the way. I would like very much to hear your feedback from it.
顺便说一下，我是那个组件的作者。我非常希望听到您的反馈。
