如何从C#中的字符串创建基于动态Lambda的Linq表达式? [英] How to create dynamic lambda based Linq expression from a string in C#?

问题描述

我在从字符串创建基于Lambda的Linq表达式时遇到了一些困难.这是我使用此示例对象/类的基本情况:

I'm having some difficulty creating Lambda-based Linq expressions from a string. Here is my basic case using this sample object/class:

public class MockClass
{
    public string CreateBy { get; set; }
}

基本上，我需要像这样转换一个字符串:

Basically I need to convert a string like this:

string stringToConvert = "x => x.CreateBy.Equals(filter.Value, StringComparison.OrdinalIgnoreCase";

插入a作为谓词/linq表达式:

Into a to predicate/linq expression:

System.Linq.Expressions.Expression<Func<T, bool>>  or in this example 
System.Linq.Expressions.Expression<Func<MockClass, bool>>

因此，它等效于以下Where方法中的Linq表达式:

So it is equivalent to the Linq expression inside of the Where method below:

query = query.Where(x => x.CreateBy.Equals(filter.Value,
StringComparison.OrdinalIgnoreCase));

我尝试使用以下帮助器，但是似乎无法弄清楚如何使它们在这种情况下工作，在这种情况下，我希望能够从事先不知道的字符串构建linq表达式: http://www.albahari.com/nutshell/predicatebuilder.aspx

I've tried using the following helpers but can't seem to figure out how to get them to work in this type of case where I want to be able to build a linq expression from string that is not know ahead of time: http://www.albahari.com/nutshell/predicatebuilder.aspx

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx (现在可以作为NuGet软件包以及称为"DynamicQuery"的软件包获得)

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx (it’s now available as a NuGet package as well called "DynamicQuery")

推荐答案

此处提出了类似的问题:

A similar question was asked here:

是否有一种简单的方法可以将(lambda表达式)字符串解析为Action委托?

据我了解，此动态查询"实际上是一个框架，用于在不使用lambda表达式的情况下传递对Where子句的限制.

As I understand it, this 'Dynamic Query' is actually a framework for passing in restrictions for a Where clause without using a lambda expression.

这样做的意义在于，lambda表达式不是动态方法，它们是匿名方法.如果您查看过一个程序集，您会发现您的lambda表达式将转换为带有任何自由变量作为字段的闭包.该类具有一个签名与您的签名相匹配的方法，在调用时分配了字段变量.

The significance of that is that lambda expressions are not dynamic methods, they're anonymous methods. If you ever take a look in an assembly, you'll see that your lambda expressions are converted into closures with any free variables as fields. The class has a method with a signature matching yours, field variables are assigned at the point of invocation.

对此进行考虑的一种好方法是，它意味着lambda表达式在编译时由c#编译器解释，并且变量是通过在运行时从此类实例化一个对象来解析的.

One good way to think about that is that it implies that your lambda expression is interpreted by the c# compiler at compile-time, and variables are resolved by instantiating an object from this class at run-time.

为证明这一点，请考虑以下因素:

To demonstrate this, consider the following:

var myLambda = x => x * x

您会注意到这是行不通的.这是因为，为了创建相关的类/方法，编译器必须在编译时知道x的类型.

You'll notice this doesn't work. That's because, in order to create the related class/method, the compiler must know, at compile-time, the type of x.

所有这些都很重要，因为lambda表达式的概念在运行时不存在于CLR(与代码中的形式相同).看起来像lambda表达式的字符串就是那个……

All of this is important because the notion of a lambda expression doesn't exist at the CLR at run-time (in the same form it is in code). A string that looks like a lambda expression is exactly that...

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