c# - 如何获取使用局部变量的 ConstantExpression 的值？

Question

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

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

string name = "Michael";

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

究竟如何从 ConstantExpression 中获取变量“name”的值？我唯一能想到的是:

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

显然这并不适合非常灵活......

一个稍微复杂一点的例子如下:

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

Answer 1

编辑:好的，由于 AHM 的评论，现在您的意思更清楚了。

基本上，代码被编译为在单独的类中捕获 name - 然后应用字段访问以从引用它的实例的常量表达式中获取其值。 (它必须这样做，因为您可以在创建表达式后更改 name 的值 - 但表达式捕获变量，而不是值。)

所以您实际上不想对 VisitConstant 中的 ConstantExpression 做任何事情 - 您想要处理 VisitMember 中的字段访问.您需要从 ConstantExpression 子项中获取值，然后将其提供给 FieldInfo 以获取值:

using System;
using System.Linq.Expressions;
using System.Reflection;

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

static class Program
{
    static void Main(string[] args)
    {
        string name = "Michael";

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

        new Visitor().Visit(exp);
    }
}

class Visitor : ExpressionVisitor    
{
    protected override Expression VisitMember
        (MemberExpression member)
    {
        if (member.Expression is ConstantExpression &&
            member.Member is FieldInfo)
        {
            object container = 
                ((ConstantExpression)member.Expression).Value;
            object value = ((FieldInfo)member.Member).GetValue(container);
            Console.WriteLine("Got value: {0}", value);
        }
        return base.VisitMember(member);
    }
}

---

编辑:好吧，稍微复杂一点的访问者类:

class Visitor : ExpressionVisitor    
{
    protected override Expression VisitMember
        (MemberExpression memberExpression)
    {
        // Recurse down to see if we can simplify...
        var expression = Visit(memberExpression.Expression);

        // If we've ended up with a constant, and it's a property or a field,
        // we can simplify ourselves to a constant
        if (expression is ConstantExpression)
        {
            object container = ((ConstantExpression) expression).Value;
            var member = memberExpression.Member;
            if (member is FieldInfo)
            {
                object value = ((FieldInfo)member).GetValue(container);
                return Expression.Constant(value);
            }
            if (member is PropertyInfo)
            {
                object value = ((PropertyInfo)member).GetValue(container, null);
                return Expression.Constant(value);
            }
        }
        return base.VisitMember(memberExpression);
    }
}

现在运行它:

var localPerson = new Person { FirstName = "Jon" };

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

Console.WriteLine("Before: {0}", exp);
Console.WriteLine("After: {0}", new Visitor().Visit(exp));

给出结果:

Before: p => Convert((p.FirstName == 
           value(Program+<>c__DisplayClass1).localPerson.FirstName))
After: p => Convert((p.FirstName == "Jon"))