动态LINQ表达式 [英] Dynamic LINQ Expression

问题描述

我正在尝试实施此

I'm trying to implement this How to specify dynamic field names in a Linq where clause? and getting a compiler error that says:

无法解析方法'Where(System.Linq.Expressions.LambdaExpression

Cannot resolve method 'Where(System.Linq.Expressions.LambdaExpression

public class Employee
{
    public string Name { get; private set; }
    public int Salary { get; private set; }

    public Employee(string name, int salary)
    {
        Name = name;
        Salary = salary;
    }
}

然后进入控制台应用程序的主要方法

Then in main method of console app

var employees = new List<Employee>
{
    new Employee("Bob", 45000),
    new Employee("Jane", 25000),
    new Employee("Jim", 5)
};

var eParam = Expression.Parameter(typeof(Employee), "e");

var comparison = Expression.Lambda(
    Expression.LessThan(
        Expression.Property(eParam, "Salary"),
        Expression.Constant(40000)),
    eParam);
var c = from e in employees.Where(comparison) // COMPILER ERROR HERE!!!
    select new {e.Name, e.Salary};

我正在使用System.Linq和System.Linq.Expressions.我在这里做什么错了?

I'm using System.Linq and System.Linq.Expressions. What am I doing wrong here?

答案是强烈键入比较变量，然后像这样调用Compile

The answer is to strongly type the comparison variable and call Compile on it like

var comparison = Expression.Lambda<Func<Employee, bool>>(
    Expression.GreaterThan(
        Expression.Property(eParam, "Salary"),
        Expression.Constant(40000)),
    eParam).Compile();

查询也可以用方法语法编写，例如

The query can also be written in method syntax like

var b = employees.Where(comparison);

除了呼叫.Compile()，还可以在员工的.Where()之前呼叫.AsQueryable().

Instead of calling .Compile(), one can call .AsQueryable() before .Where() on employees also.

推荐答案

1. 您的表达式必须为强类型:

1. Your expression has to be strongly typed:

var comparison = Expression.Lambda<Func<Employee, bool>>(... 

源必须为IQueryable.在呼叫位置之前，先呼叫列表中的AsQueryable().

Source has to be IQueryable. Call AsQueryable() on your list before calling Where.

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