如何获取使用局部变量的ConstantExpression的值？ [英] How to get the value of a ConstantExpression which uses a local variable?

问题描述

我创建了一个ExpressionVisitor实现，该实现重写VisitConstant。但是，当我创建一个使用局部变量的表达式时，似乎无法获得该变量的实际值。

I created an ExpressionVisitor implementation that overrides VisitConstant. However, when I create an expression that utilizes a local variable I can't seem to get the actual value of the variable.

public class Person
{
  public string FirstName { get; set; }
}

string name = "Michael";

Expression<Func<Person, object>> exp = p => p.FirstName == name;

我如何从ConstantExpression中获得变量 name的值？
我唯一能想到的是：

How in the world do I get the value of the variable "name" from the ConstantExpression? The only thing that I can think of is this:

string fieldValue = value.GetType().GetFields().First().GetValue(value).ToString();

显然，这并不能使其变得非常灵活....

Obviously this doesn't lend itself to being very flexible though....

一个稍微复杂的例子如下：

A slightly more complicated example would be the following:

Person localPerson = new Person { FirstName = "Michael" };
Expression<Func<Person, object>> exp = p => p.FirstName == localPerson.FirstName;

推荐答案

下面是我列出的两种情况下的解决方案。

Here's how I solved it for both cases you listed.

基本上假设'=='的右侧可以被视为不带任何参数并返回值的函数，可以将其编译为C＃委托并调用该值即可检索该值，而不必担心右侧的代码究竟会做什么。

Basically assuming that the right hand side of your '==' can be treated like a function that takes no arguments and returns a value, it can be compiled to a C# delegate and invoked to retrieve this value without worrying about exactly what the code on the right hand side does.

因此，基本示例代码在

class Visitor : ExpressionVisitor {

  protected override Expression VisitBinary( BinaryExpression node ) {

    var memberLeft = node.Left as MemberExpression;
    if ( memberLeft != null && memberLeft.Expression is ParameterExpression ) {

      var f = Expression.Lambda( node.Right ).Compile();
      var value = f.DynamicInvoke();
      }

    return base.VisitBinary( node );
    }
  }

它寻找一个二进制运算符，寻找 arg。 member == something，然后只是编译/求值右侧，对于这两个示例，您提供的结果都是字符串 Michael。

It looks for a binary op looking for "arg.member == something" then just compiles/evaluates the right hand side, and for both examples your provide the result is a string "Michael".

注意，如果您的右侧涉及使用lamda参数，例如

Note, this fails if your right hand side involved using the lamda argument like

p.FirstName == CallSomeFunc（p.FirstName）

p.FirstName == CallSomeFunc( p.FirstName )

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