创建一个可以在运行时计算的表达式

Question

我有一个产品列表，我需要根据这些产品创建表达式树，这些表达式树可以被持久化，以后可以检索和执行。这适用于客户端计算生成器。

我是 Expressions 的新手，虽然我已经阅读了相当多的文档，但这里的学习曲线有点陡峭。我想要的是能够积累 PropertyExpression 和 Operand 对开始。这是我到目前为止的内容，不确定我的结构是否正确。

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

namespace ConsoleApplication1
{
    public enum ProductType { Perishable, Fixed, Miscellaneous }    
    public enum OperandType { Addition, Sunbtraction, Multiplication, Division }

    public class Product
    {
        public string Name { get; set; }
        public ProductType Type { get; set; }
        public float Price { get; set; }
    }

    public class Configuration
    {
        public Dictionary<string, float> Dictionary { get; set; }
    }

    public class Operand
    {
        public OperandType Type { get; set; }
    }

    public class CalculationPair<TEntityType, TProperty>
        where TEntityType: class
        where TProperty: struct
    {
        public ValueTypeProperty<TEntityType, TProperty> Left { get; set; }
        public Operand Operand { get; set; }
        public ValueTypeProperty<TEntityType, TProperty> Right { get; set; }

        // How to specify TResult as an [out] parameter?
        public TResult Calculate<TResult> ()
            where TResult: struct
        {
            TResult result = default(TResult);

            if (this.Operand.Type == OperandType.Multiplication)
            {
                // How to execute the expression?
                //result = this.Left * this.Right;
            }

            return (result);
        }
    }

    public class ValueTypeProperty<TEntityType, TProperty>
        where TEntityType: class
        where TProperty: struct
    {
        public string Name { get; set; }

        public Expression<Func<TEntityType, TProperty>> PropertyExpression { get; set; }
    }

    public class ProductPriceProperty:
        ValueTypeProperty<Product, float>
    {
    }

    public static class Program
    {
        public static void Main ()
        {
            Configuration config = new Configuration();
            List<Product> products = new List<Product>();

            config.Dictionary.Add("ExportFactor", 80);
            config.Dictionary.Add("ChannelMargin", 100);

            products.Add(new Product() { Name = "1", Type = ProductType.Fixed, Price = 10 });
            products.Add(new Product() { Name = "2", Type = ProductType.Miscellaneous, Price = 20 });
            products.Add(new Product() { Name = "3", Type = ProductType.Perishable, Price = 30 });

            foreach (var product in products)
            {
                if (product.Type == ProductType.Fixed)
                {
                    CalculationPair<Product, float> calculation = new CalculationPair<Product, float>()
                    {
                        Left = new ProductPriceProperty() { Name = "Product Price", PropertyExpression = (entity => entity.Price) },
                        Operand = new Operand() { Type = OperandType.Multiplication },
                        Right = new ProductPriceProperty() { Name = "ExportFactor", PropertyExpression = (entity => config.Dictionary ["ExportFactor"]) },
                    };

                    // Calculation needs to be persisted to be retrieved later.
                    // ???!

                    // Once calculation has been reconstruction from the persistence layer, it needs to be executed.
                    product.Price = calculation.Calculate<float>();
                }
            }
        }
    }
}

更新：以下是我正在努力解决的优先级顺序：

• CalculationPair.Calculate<TReult>()函数中的表达式如何执行？
• 如何在CalculationPair.Calculate<TReult>()函数中将TResult指定为[out]参数？
• 如何将计算表达式持久化并稍后检索？

Answer 1

正如乔恩所说，您可以使用通常的表达式树，或者您可以像下面的代码一样制作闭包匿名方法：

public class CalculationPair<TEntityType, TProperty>
    where TEntityType : class
    where TProperty : struct
{
    // not sure that first three properties are needed here
    public ValueTypeProperty<TEntityType, TProperty> Left { get; set; }
    public Operand Operand { get; set; }
    public ValueTypeProperty<TEntityType, TProperty> Right { get; set; }

    // closure method
    public Func<TEntityType, TProperty> Calculator { get; set; }
}

这是使用它的 Main 方法的一部分：

foreach (var product in products)
{
    if (product.Type == ProductType.Fixed)
    {
        CalculationPair<Product, float> calculation = new CalculationPair<Product, float>()
        {
            Left = new ProductPriceProperty() { Name = "Product Price", PropertyExpression = (entity => entity.Price) },
            Operand = new Operand() { Type = OperandType.Multiplication },
            Right = new ProductPriceProperty() { Name = "ExportFactor", PropertyExpression = (entity => config.Dictionary["ExportFactor"]) },

            // only this property is needed, and it will handle reference to config object in the closure
            Calculator = (entity) => entity.Price * config.Dictionary["ExportFactor"]
        };

        // Once calculation has been reconstruction from the persistence layer, it needs to be executed.
        product.Price = calculation.Calculator(product);
    }
}

在该示例中，没有表达式树，只有常用的闭包方法。

更新1

Left 和 Right 节点表达式的问题在于，每个 this 表达式都链接到自己的 entity 参数，而不是我们创建的 ParameterExpression 并将指向真实的实体对象，所以我们需要用 ExpressionVisitor 将旧的重写为新的。它用于解析和重写需求。

这是该重写器的代码：

public class ParameterRewriter : ExpressionVisitor
{
    private readonly ParameterExpression _expToRewrite;

    public ParameterRewriter(ParameterExpression expToRewrite)
    {
        this._expToRewrite = expToRewrite;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        // we just use type checking to understand that it's our parameter, and we replace it with new one
        if (node.Type == this._expToRewrite.Type) return this._expToRewrite;
        return base.VisitParameter(node);
    }
}

这里是 CalculationPair 类：

public class CalculationPair<TEntityType, TProperty>
    where TEntityType : class
    where TProperty : struct
{
    public ValueTypeProperty<TEntityType, TProperty> Left { get; set; }
    public Operand Operand { get; set; }
    public ValueTypeProperty<TEntityType, TProperty> Right { get; set; }

    public TResult Calculate<TResult>(TEntityType entity)
        where TResult : struct
    {
        TResult result = default(TResult);

        var prop = Expression.Parameter(typeof(TEntityType), "param");
        var visitor = new ParameterRewriter(prop);
        var leftExp = visitor.Visit(Left.PropertyExpression.Body);
        var rightExp = visitor.Visit(Right.PropertyExpression.Body);

        Expression body;

        switch (this.Operand.Type)
        {
            case OperandType.Multiplication:
                body = Expression.Multiply(leftExp, rightExp);
                break;
            case OperandType.Addition:
                body = Expression.Add(leftExp, rightExp);
                break;
            case OperandType.Division:
                body = Expression.Divide(leftExp, rightExp);
                break;
            case OperandType.Sunbtraction:
                body = Expression.Subtract(leftExp, rightExp);
                break;
            default:
                throw new Exception("Unknown operand type");
        }

        var lambda = Expression.Lambda<Func<TEntityType, TResult>>(body, prop);

        // compilation is long operation, so you might need to store this Func as property and don't compile it each time
        var func = lambda.Compile();
        result = func(entity);

        return (result);
    }
}

和用法一样

product.Price = calculation.Calculate<float>(product);