表达式树-数学.马克斯替换

当我使用表达式树来替换一个方法时，例如Math。最大值，看起来它成功地在表达式树中替换了它。但是当我在实体框架中使用它时，它抛出了一个不支持数学的异常。实体框架的最大值。但是我明确地替换了它。

有人知道为什么吗?还有修复代码的方法吗?

---

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);
            }
        }
    }
}

---

如果你调用

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

它将工作。但是，如果将上面的GetTestItems()替换为实体框架_dbContext。

要测试此代码，您需要将Item结构添加到EF项目中，进行迁移，并将其推入数据库。

我希望我可以让这个问题不那么专业，这样这个问题就可以被更广泛的人回答。希望赏金足以找到答案。

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

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