表达式树 - Math.Max 替换

Question

当我使用表达式树替换方法时，例如 Math.Max，看起来它成功地替换了表达式树中的方法。但是当我在实体框架中使用它时，它会抛出一个关于实体框架不支持 Math.Max 的异常。但我明确替换它。

有人知道为什么吗？以及修复代码的方法？

---

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace ConsoleApplication1
{
    public static class CalculateDatabase
    {
        public static void Main(string[] args)
        {
            var calcs = GetCalculateToAmounts(GetTestItems(), 0.5m).ToList();
        }

        public static IQueryable<Item> GetTestItems()
        {
            var items = new List<Item>();
            items.Add(new Item()
            {
                DoNotItem = true,
                ReductionAmount = 2,
                PreviousDiscountAmount = 3,
                CurrentDiscountAmount = 10,
                CurrentAmount = 100,
                PreviousAmount = 50,
                CurrentBillAmount = 75
            });

            return items.AsQueryable();
        }

        public class Item
        {
            public bool DoNotItem { get; set; }
            public decimal ReductionAmount { get; set; }
            public decimal PreviousDiscountAmount { get; set; }
            public decimal CurrentDiscountAmount { get; set; }
            public decimal CurrentAmount { get; set; }
            public decimal PreviousAmount { get; set; }
            public decimal CurrentBillAmount { get; set; }
        }

        public static IQueryable<CalculateToAmount> GetCalculateToAmounts(this IQueryable<Item> entityItems, decimal percentage)
        {
            return entityItems.Select(CalculateAmountExpression(percentage));
        }

        public class CalcType
        { }

        public class CalculateToAmount
        {
            public CalcType CalcType { get; set; }
            public Item Item { get; set; }
            public decimal ItemAmount1 { get; set; }
            public decimal ItemAmount2 { get; set; }
            public decimal ItemAmount3 { get; set; }
            public decimal Bonus { get; set; }
            public decimal Discounts { get; set;  }
            public decimal Total { get; set; }
        }

        private static Expression<Func<Item, CalculateToAmount>> CalculateAmountExpression(this decimal percentage)
        {
            Expression<Func<Item, CalculateToAmount>> lambda = item => new CalculateToAmount()
            {
                Item = item,

                Bonus = item.DoNotItem
                    ? 0 
                    : item.CurrentBillAmount * (1 - percentage) + item.ReductionAmount,
                Discounts = item.PreviousDiscountAmount + item.CurrentDiscountAmount,
                Total = Math.Max(item.CurrentAmount + item.PreviousAmount, item.CurrentBillAmount)
            };

            var test = MathModifier.Modify(lambda);
            return test;
        }

        public class MathModifier : ExpressionVisitor
        {
            protected override Expression VisitMethodCall(MethodCallExpression node)
            {
                var isMinMethod = node.Method.Name.Equals("Min", StringComparison.InvariantCultureIgnoreCase);
                var isMaxMethod = node.Method.Name.Equals("Max", StringComparison.InvariantCultureIgnoreCase);

                if (!isMinMethod && !isMaxMethod)
                    return base.VisitMethodCall(node);

                var left = node.Arguments[0];
                var right = node.Arguments[1];

                var minMaxReplaceMethod =
                    isMinMethod
                        ? Expression.Condition(Expression.LessThan(left, right), left, right)
                        : Expression.Condition(Expression.GreaterThan(left, right), left, right);

                return minMaxReplaceMethod;
            }

            public static Expression<Func<TIn, TOut>> Modify<TIn, TOut>(Expression<Func<TIn, TOut>> expression)
            {
                var modifier = new MathModifier();
                return (Expression<Func<TIn, TOut>>)modifier.Visit(expression);
            }
        }
    }
}

---

如果你打电话

var calcs = GetCalculateToAmounts(GetTestItems(), 0.5).ToList()

它会起作用的。但是如果把上面的GetTestItems()换成实体框架_dbContext.Items就不行了。

要测试此代码，您需要将 Item 结构添加到 EF 项目，进行迁移并将其推送到数据库中。

我希望我可以减少这个技术性，以便更多人回答这个问题。希望赏金足以提供答案。如果没有，请私信我。

Answer 1

我正在完全使用 EF 6 运行您提供的代码。实体 Item 名为 Table1。代码运行成功，Math.Max的relpacement成功完成。

使用它生成的 SQL Profiler 跟踪 SQL，并针对数据库发送以下 SQL：

exec sp_executesql N'SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[DoNotItem] AS [DoNotItem], 
[Extent1].[ReductionAmount] AS [ReductionAmount], 
[Extent1].[PreviousDiscountAmount] AS [PreviousDiscountAmount], 
[Extent1].[CurrentDiscountAmount] AS [CurrentDiscountAmount], 
[Extent1].[CurrentAmount] AS [CurrentAmount], 
[Extent1].[PreviousAmount] AS [PreviousAmount], 
[Extent1].[CurrentBillAmount] AS [CurrentBillAmount], 
CASE WHEN ([Extent1].[DoNotItem] = 1) THEN cast(0 as decimal(18)) ELSE ([Extent1].[CurrentBillAmount] * (cast(1 as decimal(18)) - @p__linq__0)) + [Extent1].[ReductionAmount] END AS [C1], 
[Extent1].[PreviousDiscountAmount] + [Extent1].[CurrentDiscountAmount] AS [C2], 
CASE WHEN (([Extent1].[CurrentAmount] + [Extent1].[PreviousAmount]) > [Extent1].[CurrentBillAmount]) THEN [Extent1].[CurrentAmount] + [Extent1].[PreviousAmount] ELSE [Extent1].[CurrentBillAmount] END AS [C3]
FROM [dbo].[Table1] AS [Extent1]',N'@p__linq__0 decimal(1,1)',@p__linq__0=0.5

结论：您提供的代码有效，表达式树操作有效，您在树中的条件逻辑被转换为 SQL CASE 表达式。

可能您的 EF 测试没有足够仔细地隔离，并且 Max.Math 相关的 EF 异常来自代码的其他部分和/或间接来自触发的事件、过载等。

Answer 2

你把这个留在IQueryable土地有什么原因吗？ IQueryableland 让一切都在数据库上运行。如果您将其移至IEnumerable 土地，我认为它应该可以正常工作：

    public static IEnumerable<CalculateToAmount> GetCalculateToAmounts(this IQueryable<Item> entityItems, decimal percentage)
    {
        var func = CalculateAmountExpression(percentage).Compile();
        return entityItems.AsEnumerable().Select(func);
    }