创建具有3个条件的Lambda表达式 [英] Create a Lambda Expression With 3 conditions

问题描述

我想为此动态创建一个lambda表达式:

I want to create a lambda expression dynamically for this:

(o => o.Year == year && o.CityCode == cityCode && o.Status == status)

我这样写:

var body = Expression.AndAlso(
               Expression.Equal(
                    Expression.PropertyOrField(param, "Year"),
                    Expression.Constant(year)
               ),
               Expression.Equal(
                    Expression.PropertyOrField(param, "CityCode"),
                    Expression.Constant(cityCode)
               )
               ,
               Expression.Equal(
                    Expression.PropertyOrField(param, "Status"),
                    Expression.Constant(status)
               )
           );

但是对于这段代码:

Expression.Equal(
                    Expression.PropertyOrField(param, "Status"),
                    Expression.Constant(status)
                )

我遇到一个错误:

无法从"System.Linq.Expressions.BinaryExpression"转换为"System.Reflection.MethodInfo"

Cannot convert from 'System.Linq.Expressions.BinaryExpression' to 'System.Reflection.MethodInfo'

如何为lambda表达式添加3个条件?

How I can add 3 conditions to a lambda expression?

推荐答案

Expression.AndAlso采用两个表达式.有一个带有三个参数的重载，但第三个参数是对两个操作数实现和操作的方法的MethodInfo(对于AndAlso它不允许覆盖真实性的详细信息，因此第一个操作数仍将需要具有true和false运算符或可转换为bool).

Expression.AndAlso takes two expressions. There is an overload that takes three arguments, but that third argument is a MethodInfo of a method that implements an and operation on the two operands (there are further restrictions in the case of AndAlso as it doesn't allow the details of truthiness to be overridden, so the first operand will still need to either have a true and false operator or be castable to bool).

所以您想要的就是:

(o => o.Year == year && (o.CityCode == cityCode && o.Status == status))

将会是:

var body = Expression.AndAlso(
    Expression.Equal(
        Expression.PropertyOrField(param, "Year"),
        Expression.Constant(year)
    ),
    Expression.AndAlso(
        Expression.Equal(
            Expression.PropertyOrField(param, "CityCode"),
            Expression.Constant(cityCode)
        ),
        Expression.Equal(
            Expression.PropertyOrField(param, "Status"),
            Expression.Constant(status)
        )
    )
);

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