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)

我是这样写的:

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)
                )

我有一个错误:

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

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

如何向 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 需要两个表达式.有一个带有三个参数的重载，但第三个参数是一个方法的 MethodInfo，该方法在两个操作数上实现了 and 操作(在这种情况下还有进一步的限制AndAlso 因为它不允许覆盖真实性的细节，所以第一个操作数仍然需要有 true 和 false> 运算符或可强制转换为 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).

所以你想要的是:

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

这将是:

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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